Optical information recording/reproducing apparatus

ABSTRACT

An optical information recording/reproducing apparatus records information on and/or reproduces information from an optical recording medium based on a reflected light beam from the optical recording medium. The apparatus includes a light splitting part having a plurality of light splitting stages which split the reflected light beam from the optical recording medium into a plurality of light beams, and a photodetector unit including a plurality of photodetectors which receive the plurality of light beams from the light splitting part. The photodetector unit includes a plurality of photodetectors which receive light beams used to detect a focal error, at least one photodetector which receives a light beam used to detect a tracking error, and a plurality of photodetectors which receive light beams used to detect magneto-optic information recorded on the optical recording medium.

This application is a Continuation-In-Part application of a U.S. patentapplication Ser. No. 08/742,764 filed Nov. 1, 1996 (now U.S. Pat. No.5,793,725) which is a Divisional Application of a U.S. patentapplication Ser. No. 08/513,578 filed Aug. 10, 1995 (now U.S. Pat. No.5,623,462 issued Apr. 22, 1997) which is a File-Wrapper-ContinuationApplication of a U.S. patent application Ser. No. 08/084,362 filed Jun.30, 1993 (now abandoned).

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to optical informationrecording/reproducing apparatuses, and more particularly to an opticalinformation recording/reproducing apparatus which optically recordsinformation on a recording medium and/or optically reproduces theinformation from the recording medium.

2. Description of the Related Art

An optical disk unit is an example of a unit which uses an opticalinformation recording/reproducing apparatus. The optical disk unit canbe used as a storage unit of a file system or the like, and is suitedfor storing programs and large amounts of data. In such an optical diskunit, it is desirable that an optical system thereof can accuratelyrecord and/or reproduce the information, and that the number of partsthereof is minimized so as to reduce the cost of the optical disk unitas a whole.

Various techniques have been proposed to detect a focal error in theoptical disk unit. Generally, the astigmatism technique and the Foucaulttechnique are well known. The Foucault technique is sometimes alsoreferred to as the double knife edge technique.

Compared to the astigmatism technique, the Foucault technique is lessaffected by the external disturbance that occurs when a track on anoptical disk is traversed, the birefringence of the optical disk.Accordingly, the mixture of the external disturbance into a focal errorsignal when the Foucault technique is employed is extremely smallcompared to the case where the astigmatism technique is employed. Inaddition, the Foucault technique detects a reflected light beam from theoptical disk by a photodetector which is arranged in a vicinity of animage formation point of the optical beam, and for this reason, anabnormal offset is unlikely generated in the focal error signal even ifthe reflected light beam shifts from an optical axis. Because of theseadvantageous features obtainable by the Foucault technique, it isdesirable to employ the Foucault technique as the focal error detectiontechnique.

First, an example of an optical information recording/reproducingapparatus within a conventional magneto-optic disk unit which employsthe Foucault technique will be described with reference to FIG. 1.

In an optical system of the optical information recording/reproducingapparatus shown in FIG. 1, a laser beam which is emitted from a laserdiode 201 is formed into a parallel beam having an oval cross section ina collimator lens 202, and is thereafter formed into a light beam havinga circular cross section in a true circle correction prism 203. Thelight beam from the true circle correction prism. 203 is transmittedthrough a beam splitter 204, reflected by a mirror 205, and is convergedon a disk 207 via an objective lens 206. A reflected light beam from thedisk 207 enters the beam splitter 204 via the objective lens 206 and themirror 205, but this time the reflected light beam is reflected by thebeam splitter 204 and is directed towards a beam splitter 208. The beamsplitter 208 splits the reflected light beam into two light beams, andsupplies one light beam to a magneto-optic signal detection system andthe other light beam to a servo signal detection system.

The magneto-optic signal detection system includes a Wollaston prism209, a lens 210 and a 2-part photodetector 211. One of the two lightbeams output from the beam splitter 208 is input to the 2-partphotodetector 211 via the Wollaston prism 209 and the lens 210, and the2-part photodetector 211 detects the magneto-optic signal, that is, theinformation signal, based on the input light beam.

The servo signal detection system includes a condenser lens 212, a beamsplitter 213, a 2-part photodetector 214, a composite prism 215 and a4-part photodetector 216. The other of the two light beams output fromthe beam splitter 208 is input to the 2-part photodetector 214 via thecondenser lens 212 and the beam splitter 213 on one hand, and is inputto the 4-part photodetector 216 via the composite prism 215 on theother. The 2-part photodetector 214 forms a tracking error detectionsystem in the servo signal detection system, and generates a trackingerror signal by obtaining a difference between the outputs of the 2-partphotodetector 214 according to the push-pull technique. The compositeprism 215 and the 4-part photodetector 216 form a focal error detectionsystem in the servo signal detection system, and generates a focal errorsignal based on outputs of the 4-part photodetector 216 according to theFoucault technique. A focus servo operation controls the relativepositional relationship of the objective lens 206 and the disk 207 basedon the focal error signal, so that an in-focus position is located onthe disk 207.

Next, a description will be given of the push-pull technique, byreferring to FIGS. 2 and 3. FIGS. 2(a), 2(b) and 2(c) show the relativepositional relationship of the light beam which is irradiated via theobjective lens 206 and the track on the disk 207, and FIGS. 3(a), 3(b)and 3(c) show a spot of the reflected light beam which is formed on the2-part photodetector 214 in correspondence with FIGS. 2(a), 2(b) and2(c).

FIG. 2(b) shows a case where the spot of the light beam is positioned atthe center of a guide groove 207 a of the disk 207. In this case, thespot of the reflected light beam on the 2-part photodetector 214 isformed as shown in FIG. 3(b), and a light intensity distribution b issymmetrical to the right and left. If the outputs of the 2-partphotodetector 214 are denoted by A and B, a tracking error signal TES isgenerated based on the following formula (1).

TES=A−B   (1)

In this case, the tracking error signal TES is 0.

If the spot of the light beam in FIG. 2(b) shifts to the right as shownin FIG. 2(a), a light intensity distribution a of the reflected lightbeam becomes unbalanced and the light intensity at the left detectorpart of the 2-part photodetector 214 becomes larger as shown in FIG.3(a). For this reason, the tracking error signal TES in this case takesa positive value.

On the other hand, if the spot of the light beam in FIG. 2(b) shifts tothe left as shown in FIG. 2(c), a light intensity distribution c of thereflected light beam becomes unbalanced and the light intensity at theright detector part of the 2-part photodetector 214 becomes larger asshown in FIG. 3(c). For this reason, the tracking error signal TES inthis case takes a negative value.

Accordingly, if the spot of the light beam on the disk 207 shifts to theright or left with respect to the central position of the guide groove207 a, the tracking error signal TES which is obtained in the abovedescribed manner changes to a more positive or negative value. Thus, itis possible to carry out an appropriate tracking control operation basedon the tracking error signal TES.

FIG. 4 shows an example of the shapes of the composite prism 215 and the4-part photodetector 216. The 4-part photodetector 216 includes detectorparts 216 a, 216 b, 216 c and 216 d. A focal error signal FES isgenerated from outputs A, B, C and D respectively output from thedetector parts 216 a, 216 b, 216 c and 216 d of the 4-part photodetector216, based on the following formula (2).

FES=(A−B)+(C−D)   (2)

Ideally, the focal error signal FES is 0 in a state where the spot ofthe light beam is in focus on the disk 207. In this case, the focalerror signal FES having an S-curve as shown in FIG. 5 is obtaineddepending on the distance between the objective lens 206 and the disk207. In FIG. 5, the ordinate indicates the focal error signal FES, andthe abscissa indicates the distance between the objective lens 206 andthe disk 207. The origin (0) on the abscissa corresponds to the in-focusposition, and the above distance becomes smaller towards the left andlarger towards the right in FIG. 5.

FIGS. 6(a), 6(b) and 6(c) show the relative positional relationship ofthe objective lens 206 and the disk 207. FIG. 6(a) shows a case wherethe objective lens 206 is close to the disk 207 and the in-focusposition is located above the disk 207 in the figure, FIG. 6(b) shows acase where the in-focus position is located on the disk 207, and FIG.6(c) shows a case where the objective lens 296 is far from the disk 207and the in-focus position is located between the disk 207 and theobjective lens 206 in the figure.

FIGS. 7(a), 7(b) and 7(c) show beam spots on the 4-part photodetector216 for each relative positional relationship of the objective lens 206and the disk 207 shown in FIGS. 6(a), 6(b) and 6(c). FIG. 7(a) shows thebeam spots for the positional relationship shown in FIG. 6(a), FIG. 7(b)shows the beam spots for the in-focus positional relationship shown inFIG. 6(b), and FIG. 7(c) shows the beam spots for the positionalrelationship shown in FIG. 6(c). As shown in FIG. 7(b), the beam spotson the 4-part photodetector 216 have oval shapes in the in-focusposition, and a division line E of the 4-part photodetector 216 ispositioned at the center of each oval beam spot.

However, in the actual disk unit, the distribution of the quantity ofthe light beam irradiated on the disk 207 may be unbalanced, and errorsmay exist in the mounting positions of the composite prism 215 and the4-part photodetector 216.

The light intensity distribution of the light beam which is emitted fromthe laser diode 201 can generally be approximated by a Gaussiandistribution. Hence, if the optical axis of the light beam emitted fromthe laser diode 201 matches the optical axes of other optical parts, itis possible to obtain a Gaussian distribution in which the center of thelight intensity of the light beam input to the objective lens 206matches the optical axis (point 0) shown in FIG. 8. However, if thelight beam emitted from the laser diode 201 is inclined by an angle θ inFIG. 1, the center of the light intensity of the light beam input to theobjective lens 206 is shifted from the optical axis (point 0) in theGaussian distribution as indicated by a dotted line in FIG. 8. The“unbalanced distribution” of the light quantity of the light beamirradiated on the disk 207 or “decentering”, refers to such a differencebetween the optical axis and the center of light beam intensitydistribution.

On the other hand, the “mounting error” of the composite prism 215, forexample, refers to a positional error of the composite prism 215 in ay-direction in FIG. 4. If such a mounting error exists, the compositeprism 215 cannot accurately split the incident light beam into two equallight beams. Generally, if the division line E of the composite prism215 shifts a distance Δy in the y-direction from the center of theincident light beam, where the division line E extends in thex-direction in FIG. 4, the value of the mounting error can be obtainedfrom [Δy/(diameter of light beam)]·100 (%).

For this reason, if the quantity of the light beam which is split intotwo in the composite prism 215 changes and a positional error of thedivision line E of the 4-part photodetector 216 occurs, a focal offsetis generated. The generation of the “focal offset” means that the focalerror signal FES described by the formula (2) becomes 0 at a positionother than the in-focus position. Thus, according to the conventionalFoucault technique, the tolerable margin of the focal error detectionsystem is extremely small with respect to the unbalanced distribution ofthe quantity of light beam irradiated on the disk 207, the mountingerror of the composite prism 215 and the 4-part photodetector 216 andthe like. Therefore, there is a problem in that it is extremelydifficult to obtain an accurate focal error signal due to the aboveerror factors.

SUMMARY OF THE INVENTION

Accordingly, it is a general object of the present invention to providea novel and useful optical information recording/reproducing apparatusin which the problem described above is eliminated.

Another and more specific object of the present invention is to providean optical information recording/reproducing apparatus which recordsinformation on and/or reproduces information from an optical recordingmedium and detects a focal error based on a reflected light beam fromthe optical recording medium, comprising a composite prism deflecting apart of the reflected light beam to at least two positions excluding acentral part of the reflected light beam, and photodetector meansincluding a plurality of photodetectors for respectively detecting thedeflected parts of the reflected light beam and outputting detectionoutputs, where the focal error is detected based on the detectionoutputs of the photodetector means. According to the optical informationrecording/reproducing apparatus of the present invention, it is possibleto obtain an accurate focal error signal because the tolerable margin ofthe focal error detection system can be set large with respect to theunbalanced distribution of the quantity of the light beam irradiated onthe optical recording medium, the mounting error of the composite prism,the photodetector and the like.

Still another object of the present invention is to provide an opticalinformation recording/reproducing apparatus which records information onand/or reproduces information from an optical recording medium anddetects a tracking error and a focal error based on a reflected lightbeam from the optical recording medium, comprising beam splitter meansfor splitting the reflected light beam into at least one first beamwhich is used for detecting the tracking error and at least two secondbeams which are used for detecting the focal error, and photodetectormeans including a first photodetector which detects the first beam at aposition other than an image formation point of the first beam, andsecond photodetectors for detecting the second beams approximately atimage formation points of the second beams. According to the opticalinformation recording/reproducing apparatus of the present invention, itis unnecessary to provide two independent optical paths even if thefocal error is to be detected according to the Foucault technique andthe tracking error is to be detected according to the push-pulltechnique. As a result, it is possible to reduce the space occupied bythe optical system within the optical information recording/reproducingapparatus, and to reduce the number of required parts. For this reason,it is possible to reduce both the size and cost of the opticalinformation recording/reproducing apparatus and an optical disk unit towhich the optical information recording/reproducing apparatus may beapplied.

A further object of the present invention is to provide an opticalinformation recording/reproducing apparatus which records information onand/or reproduces information from an optical recording medium anddetects a focal error and a tracking error based on a reflected lightbeam from the optical recording medium, comprising beam splitter meansfor splitting the reflected light beam into first through fourth lightbeams which propagate generally in a predetermined direction, andphotodetector means for detecting the focal error in response to thefirst and second light beams, and for detecting the tracking error inresponse to the third and fourth light beams. According to the opticalinformation recording/reproducing apparatus of the present invention, itis possible to improve the reliability of the focal error detection andtracking error detection. In addition, it is possible to reduce both thesize and cost of the optical information recording/reproducing apparatusand an optical disk unit to which the optical informationrecording/reproducing apparatus may be applied.

Another object of the present invention is to provide an opticalinformation recording/reproducing apparatus which records an informationsignal on and/or reproduces the information signal from an opticalrecording medium and detects a tracking error, a focal error, theinformation signal and an address signal based on a reflected light beamfrom the optical recording medium, comprising beam splitter means forsplitting the reflected light beam into first through sixth light beamswhich propagate generally in a predetermined direction, andphotodetector means for detecting the focal error in response to thefirst and second light beams, and for detecting the tracking error, theinformation signal and the address signal in response to the thirdthrough sixth light beams. According to the optical informationrecording/reproducing apparatus of the present invention, it is possibleto detect the focal error signal, the tracking error signal, theinformation signal and the address signal by the single photodetectormeans. For this reason, it is possible to detect all of the necessarysignals using a single optical path to the beam splitter means and thesingle photodetector means. Hence, it is possible to reduce both thesize and cost of the optical information recording/reproducing apparatusand an optical disk unit to which the optical informationrecording/reproducing apparatus may be applied.

Still another object of the present invention is to provide an opticalinformation recording/reproducing apparatus which records information onand/or reproduces information from an optical recording medium based ona reflected light beam from the optical recording medium, comprising alight splitting part having a plurality of light splitting stages whichsplit the reflected light beam from the optical recording medium into aplurality of light beams, and a photodetector unit including a pluralityof photodetectors which receive the plurality of light beams from thelight splitting part, where the photodetector unit includes a pluralityof photodetectors which receive light beams used to detect a focalerror, at least one photodetector which receives a light beam used todetect a tracking error, and a plurality of photodetectors which receivelight beams used to detect magneto-optic information recorded on theoptical recording medium. According to the optical informationrecording/reproducing apparatus of the present invention, the number ofoptical parts forming the apparatus can be effectively reduced, therebysimplifying the production process and reducing the cost of theapparatus. Further, it is possible to suppress noise and obtain a focalerror signal, a tracking error signal and a magneto-optic signal havinga high signal quality.

Other objects and further features of the present invention will beapparent from the following detailed description when read inconjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing an example of a conventional opticalinformation recording/reproducing apparatus;

FIGS. 2(a), 2(b) and 2(c) are diagrams showing the relative positionalrelationship between a light beam which is irradiated via an objectivelens and a track on an optical disk for explaining the push-pulltechnique;

FIGS. 3(a), 3(b) and 3(c) are diagrams showing a spot of a reflectedlight beam which is formed on a 2-part photodetector;

FIG. 4 is a perspective view showing an example of the shapes of acomposite prism and a 4-part photodetector;

FIG. 5 is a diagram showing the relationship of a distance between theobjective lens and the disk and a focal error signal FES;

FIGS. 6(a), 6(b) and 6(c) are diagrams showing the relative positionalrelationship of the objective lens and the disk;

FIGS. 7(a), 7(b) and 7(c) are diagrams showing a spot of a reflectedlight beam which is formed on the 4-part photodetector;

FIG. 8 is a diagram showing a Gaussian distribution;

FIG. 9 is a perspective view showing an important part of a firstembodiment of an optical information recording/reproducing apparatusaccording to the present invention;

FIG. 10 is a perspective view showing an important part of a secondembodiment of the optical information recording/reproducing apparatusaccording to the present invention;

FIG. 11 is a perspective view showing an important part of a thirdembodiment of the optical information recording/reproducing apparatusaccording to the present invention;

FIG. 12 is a perspective view showing an important part of a fourthembodiment of the optical information recording/reproducing apparatusaccording to the present invention;

FIGS. 13(a), 13(b), 13(c) and 13(d) are diagrams showing simulationresults describing the relationship of a focus position and a focalerror signal FES in the prior art;

FIGS. 14(a), 14(b), 14(c) and 14(d) are diagrams showing simulationresults describing the relationship of the focus position and the focalerror signal FES in the first or third embodiment;

FIG. 15 is a diagram showing the relationship of a detector shift and afocal offset in the prior art;

FIG. 16 is a diagram showing the relationship of the detector shift andthe focal offset in the first or third embodiment;

FIG. 17 is a diagram showing a fifth embodiment of the opticalinformation recording/reproducing apparatus according to the presentinvention;

FIGS. 18(a) and 18(b) are diagrams showing a composite prism of thefifth embodiment on an enlarged scale;

FIG. 19 is a perspective view showing an important part of the fifthembodiment;

FIGS. 20(a) and 20(b) are diagrams showing a composite prism of a sixthembodiment of the optical information recording/reproducing apparatusaccording to the present invention on an enlarged scale;

FIG. 21 is a perspective view showing an important part of the sixthembodiment;

FIGS. 22(a) and 22(b) are diagrams showing a composite prism of aseventh embodiment of the optical information recording/reproducingapparatus according to the present invention on an enlarged scale;

FIG. 23 is a perspective view showing an important part of the seventhembodiment;

FIG. 24 is a diagram showing an eighth embodiment of the opticalinformation recording/reproducing apparatus according to the presentinvention;

FIG. 25 is a perspective view showing an important part of the eighthembodiment;

FIG. 26 is a plan view showing a photodetector unit of the eighthembodiment;

FIG. 27 is a perspective view showing an important part of a firstmodification of the eighth embodiment of the optical informationrecording/reproducing apparatus according to the present invention;

FIG. 28 is a plan view showing a photodetector unit of the firstmodification of the eighth embodiment;

FIG. 29 is a perspective view showing an important part of a secondmodification of the eighth embodiment of the optical informationrecording/reproducing apparatus according to the present invention;

FIG. 30 is a perspective view showing a composite prism of the secondmodification of the eighth embodiment;

FIG. 31 is a plan view showing a photodetector unit of the secondmodification of the eighth embodiment;

FIG. 32 is a perspective view showing an important part of a ninthembodiment of the optical information recording/reproducing apparatusaccording to the present invention;

FIG. 33 is a cross sectional view showing an important part of aholographic optical element of the ninth embodiment;

FIG. 34 is a perspective view for explaining the functions of theholographic optical element by itself;

FIG. 35 is a plan view for explaining the construction of theholographic optical element;

FIG. 36 is a cross sectional view showing an important part of aholographic optical element of a tenth embodiment of the opticalinformation recording/reproducing apparatus according to the presentinvention;

FIG. 37 is a perspective view for explaining desirable functions of theholographic optical element by itself;

FIG. 38 is a diagram showing an eleventh embodiment of the opticalinformation recording/reproducing apparatus according to the presentinvention;

FIGS. 39(a) and 39(b) are diagrams showing a composite prism of theeleventh embodiment;

FIG. 40 is a perspective view showing an important part of the eleventhembodiment;

FIG. 41 is a diagram showing a twelfth embodiment of the opticalinformation recording/reproducing apparatus according to the presentinvention;

FIG. 42 is a perspective view showing a Wollaston prism of the twelfthembodiment;

FIG. 43 is a diagram showing an integral part made up of a compositeprism and the Wollaston prism of the twelfth embodiment;

FIG. 44 is a perspective view showing an important part of the twelfthembodiment;

FIG. 45 is a diagram showing a thirteenth embodiment of the opticalinformation recording/reproducing apparatus according to the presentinvention;

FIG. 46 is a perspective view showing an important part of thethirteenth embodiment;

FIG. 47 is a plan view showing a photodetector unit in a state where anobjective lens and a magneto-optic disk are close to each other;

FIG. 48 is a plan view showing the photodetector unit in a state where alaser beam is in focus on the magneto-optic disk;

FIG. 49 is a plan view showing the photodetector unit in a state wherethe objective lens and the magneto-optic disk are far away from eachother;

FIGS. 50A, 50B and 50C respectively are diagrams for explaining spotsformed on photodetectors of the photodetector unit by light beams outputfrom a composite prism;

FIG. 51 is a plan view showing the photodetector unit in a hightemperature state;

FIG. 52 is a plan view showing a photodetector unit used in a firstmodification of the thirteenth embodiment of the optical informationrecording/reproducing apparatus according to the present invention;

FIG. 53 is a plan view showing a photodetector unit used in a secondmodification of the thirteenth embodiment of the optical informationrecording/reproducing apparatus according to the present invention;

FIG. 54 is a perspective view showing an important part of a thirdmodification of the thirteenth embodiment of the optical informationrecording/reproducing apparatus according to the present invention;

FIG. 55 is a perspective view showing a composite prism of the thirdmodification of the thirteenth embodiment;

FIG. 56 is a plan view showing a photodetector unit of the thirdmodification of the thirteenth embodiment;

FIG. 57 is a perspective view showing an important part of a fourteenthembodiment of the optical information recording/reproducing apparatusaccording to the present invention;

FIG. 58 is a perspective view showing an important part of a fifteenthembodiment of the optical information recording/reproducing apparatusaccording to the present invention;

FIG. 59 is a perspective view showing an important part of an sixteenthembodiment of the optical information recording/reproducing apparatusaccording to the present invention;

FIGS. 60A through 60D respectively are circuit diagrams showing circuitsfor obtaining a focal error signal, a tracking error signal, amagneto-optic signal and an identification signal of the firstmodification of the eighth embodiment; and

FIG. 61 is a perspective view showing a composite prism which may beused in place of the composite prisms shown in FIGS. 30 and 55.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 9 is a perspective view showing an important part of a firstembodiment of an optical information recording/reproducing apparatusaccording to the present invention. A composite prism 15 includestapered parts 15 a and 15 b, and a central part 15 c having no taper. Onthe other hand, a 4-part photodetector unit 16 includes 2-partphotodetectors 16 a and 16 b, and a central part 16 c which includes nophotodetector part. The composite prism 15 and the 4-part photodetectorunit 16 are provided in place of the composite prism 215 and the 4-partphotodetector 216 in the optical system of the optical informationrecording/reproducing apparatus shown in FIG. 1, for example, and detectthe focal error.

The reflected light beam which is obtained via the beam splitters 204and 208, the condenser lens 212 and the beam splitter 213 is input tothe composite prism 15. Out of the reflected light beam which is inputto the composite prism 15, the light beams transmitted through thetapered parts 15 a and 15 b of the composite prism 15 form spots on thecorresponding 2-part photodetectors 16 a and 16 b of the 4-partphotodetector unit 16. Accordingly, by carrying out the operation of theformula (2) described above using the outputs of the 2-partphotodetectors 16 a and 16 b, it is possible to obtain a focal errorsignal FES similarly to the conventional case.

On the other hand, out of the reflected light beam, the light beam whichis transmitted through the central part 15 c of the composite prism 15is input to the central part 16 c of the 4-part photodetector unit 16.As a result, out of the reflected light beam input to the compositeprism 15, the light beam which is transmitted through the central part15 c of the composite prism 15 is not input to the 2-part photodetectors16 a and 16 b of the 4-part photodetector unit 16, that is, not input toa light sensitive part of the 4-part photodetector unit 16.

In this embodiment, the spots which are formed on the 2-partphotodetectors 16 a and 16 b of the 4-part photodetector unit 16 haveoval shapes with a major axis greater than that of the conventionalcase. In other words, the oval spots are longer in a directionperpendicular to the division line E of each of the 2-partphotodetectors 16 a and 16 b. For this reason, the focal offset which isgenerated by the positional error of the division lines E is extremelysmall.

Next, a description will be given of a second embodiment of the opticalinformation recording/reproducing apparatus according to the presentinvention, by referring to FIG. 10. FIG. 10 is a perspective viewshowing an important part of the second embodiment.

In FIG. 10, a composite prism 25 has a trapezoidal column shape andincludes tapered parts 25 a and 25 b, and a central part 25 c which hasno taper. On the other hand, a 4-part photodetector unit 26 includes2-part photodetectors 26 a and 26 b, and a central part 26 c whichincludes no photodetector part. The composite prism 25 and the 4-partphotodetector unit 26 are provided in place of the composite prism 215and the 4-part photodetector 216 in the optical system of the opticalinformation recording/reproducing apparatus shown in FIG. 1, forexample, and detect the focal error.

The reflected light beam which is obtained via the beam splitters 204and 208, the condenser lens 212 and the beam splitter 213 is input tothe composite prism 25. Out of the reflected light beam which is inputto the composite prism 25, the light beams transmitted through thetapered parts 25 a and 25 b of the composite prism 25 form spots on thecorresponding 2-part photodetectors 26 a and 26 b of the 4-partphotodetector unit 26. Accordingly, by carrying out the operation of theformula (2) described above using the outputs of the 2-partphotodetectors 26 a and 26 b, it is possible to obtain a focal errorsignal FES similarly to the conventional case.

On the other hand, out of the reflected light beam, the light beam whichis transmitted through the central part 25 c of the composite prism 25is input to the central part 26 c of the 4-part photodetector unit 26.As a result, out of the reflected light beam input to the compositeprism 25, the light beam which is transmitted through the central part25 c of the composite prism 25 is not input to the 2-part photodetectors26 a and 26 b of the 4-part photodetector unit 26, that is, not input toa light sensitive part of the 4-part photodetector unit 26.

In this embodiment, the spots which are formed on the 2-partphotodetectors 26 a and 26 b of the 4-part photodetector unit 26 haveoval shapes with a major axis greater than that of the conventionalcase. In other words, the oval spots are longer in a directionperpendicular to the division line E of each of the 2-partphotodetectors 26 a and 26 b. For this reason, the focal offset which isgenerated by the positional error of the division lines E is extremelysmall.

Next, a description will be given of a third embodiment of the opticalinformation recording/reproducing apparatus according to the presentinvention, by referring to FIG. 11. FIG. 11 is a perspective showing animportant part of the third embodiment. In FIG. 11, those parts whichare the same as those corresponding parts in FIG. 9 are designated bythe same reference numerals, and a description thereof will be omitted.

In this embodiment, a light absorbing or blocking layer 15 cA is formedon the central part 15 c of a composite prism 15A so as to absorb orblock the light beam which has the wavelength of the light emitted fromthe laser diode 201 shown in FIG. 1. This light absorbing or blockinglayer 15 cA may be formed on the front surface or the rear surface ofthe composite prism 15A at the central part 15 c. In addition, thisembodiment uses the same 4-part photodetector 216 used in theconventional case shown in FIG. 1.

In this case, the reflected light beam which is obtained via the beamsplitters 204 and 208, the condenser lens 212 and the beam splitter 213is input to the composite prism 15A. Out of the reflected light beamwhich is input to the composite prism 15A, the light beams transmittedthrough the tapered parts 15 a and 15 b of the composite prism 15A formspots on the corresponding detector parts 216 a, 216 b, 216 c and 216 dof the 4-part photodetector 216. Accordingly, by carrying out theoperation of the formula (2) described above using the outputs of thedetector parts 216 a, 216 b, 216 c and 216 d, it is possible to obtain afocal error signal FES similarly to the conventional case.

On the other hand, out of the reflected light beam, the light beam whichis input to the central part 15 c of the composite prism 15A is absorbedor blocked light absorbing or blocking layer 15 cA and will not be inputto the 4-part photodetector 216. As a result, out of the reflected lightbeam input to the composite prism 15A, the light beam which is input tothe central part 15 c of the composite prism 15A is not input to thedetector parts 216 a, 216 b, 216 c and 216 d of the 4-part photodetector216, that is, not input to a light sensitive part of the 4-partphotodetector 216.

In this embodiment, the spots which are formed on the detector parts 216a, 216 b, 216 c and 216 d of the 4-part photodetector 216 have ovalshapes with a major axis greater than that of the conventional case. Inother words, the oval spots are longer in a direction perpendicular tothe division line E of the 4-part photodetector 216. For this reason,the focal offset which is generated by the positional error of thedivision line E is extremely small.

Next, a description will be given of a fourth embodiment of the opticalinformation recording/reproducing apparatus according to the presentinvention, by referring to FIG. 12. FIG. 12 is a perspective showing animportant part of the fourth embodiment. In FIG. 12, those parts whichare the same as those corresponding parts in FIG. 10 are designated bythe same reference numerals, and a description thereof will be omitted.

In this embodiment, a light absorbing or blocking layer 25 cA is formedon the central part 25 c of a composite prism 25A which has a triangularprism shape. so as to absorb or block the light beam which has thewavelength of the light emitted from the laser diode 201 shown in FIG.1. This light absorbing or blocking layer 25 cA may be formed on thefront surface or the rear surface of the composite prism 25A at thecentral part 25 c. In addition, this embodiment uses a 4-partphotodetector 216 A shown in FIG. 12.

In this case, the reflected light beam which is obtained via the beamsplitters 204 and 208, the condenser lens 212 and the beam splitter 213is input to the composite prism 25A. Out of the reflected light beamwhich is input to the composite prism 25A, the light beams transmittedthrough the tapered parts 25 a and 25 b of the composite prism 25A formspots on the corresponding detector parts 216 a, 216 b, 216 c and 216 dof the 4-part photodetector 216A. Accordingly, by carrying out theoperation of the formula (2) described above using the outputs of thedetector parts 216 a, 216 b, 216 c and 216 d, it is possible to obtain afocal error signal FES similarly to the conventional case.

On the other hand, out of the reflected light beam, the light beam whichis transmitted through the central part 25 c of the composite prism 25Ais absorbed or blocked light absorbing or blocking layer 25 cA and willnot be input to the 4-part photodetector 216A. As a result, out of thereflected light beam input to the composite prism 25A, the light beamwhich is input to the central part 25 c of the composite prism 25A isnot input to the detector parts 216 a, 216 b, 216 c and 216 d of the4-part photodetector 216A, that is, not input to a light sensitive partof the 4-part photodetector 216A.

In this embodiment, the spots which are formed on the detector parts 216a, 216 b, 216 c and 216 d of the 4-part photodetector 216A have ovalshapes with a major axis greater than that of the conventional case. Inother words, the oval spots are longer in a direction perpendicular tothe division lines E of the 4-part photodetector 216A. For this reason,the focal offset which is generated by the positional error of thedivision lines E is extremely small.

FIGS. 13(a) through 13(d) show simulation results describing therelationship of the focal position and the focal error signal FES in theprior art shown in FIG. 4. In FIGS. 13(a) through 13(d), a bold solidline indicates a case where a detector shift is 0, a solid lineindicates a case where the detector shift is +10 μm, a dotted lineindicates a case where the detector shift is +20 μm, a bold dotted lineindicates a case where the detector shift is −10 μm, and a bold and finedotted line indicates a case where the detector shift is −20 μm. The“detector shift” refers to the shift of the division line E of the4-part photodetector 216 in the y-direction in FIG. 4, and an upwardshift in FIG. 4 is taken as a positive (+) shift and a downward shift inFIG. 4 is taken as a negative (−) shift.

FIG. 13(a) shows a case where the mounting error of the composite prism215 is 5%, FIG. 13(b) shows a case where the mounting error is 10%, FIG.13(c) shows a case where the inclination angle θ of the light beamemitted from the laser diode 201 is 0.5°, and FIG. 13(d) shows a casewhere the inclination angle θ of the light beam emitted from the laserdiode 201 is 1.0°. The case where the inclination angle θ is 0.5°corresponds to the case where the shift of the light beam from theoptical axis at the objective lens 206 is 0.25 mm, and the case wherethe inclination angle θ is 1.0° corresponds to the case where the shiftof the light beam from the optical axis at the objective lens 206 is0.50 mm. Accordingly, if the detector shift indicated by the dotted linein FIG. 13(a) is +20 μm, for example, it may be seen that a focal offsetof approximately 2.0 μm is generated.

On the other hand, FIGS. 14(a) through 14(d) show simulation resultsdescribing the relationship of the focal position and the focal errorsignal FES in the first embodiment shown in FIG. 9 or the thirdembodiment shown in FIG. 11. In FIGS. 14(a) through 14(d), a bold solidline indicates a case where the detector shift is 0, a solid lineindicates a case where the detector shift is +10 μm, a dotted lineindicates a case where the detector shift is +20 μm, a bold dotted lineindicates a case where the detector shift is −10 μm, and a bold and finedotted line indicates a case where the detector shift is −20 μm.

FIG. 14(a) shows a case where the mounting error of the composite prism15 or 15A is 5%, FIG. 14(b) shows a case where the mounting error is10%, FIG. 14(c) shows a case where the inclination angle θ of the lightbeam emitted from the laser diode 201 is 0.5°, and FIG. 14(d) shows acase where the inclination angle θ of the light beam emitted from thelaser diode 201 is 1.0°. The case where the inclination angle θ is 0.5°corresponds to the case where the shift of the light beam from theoptical axis at the objective lens 206 is 0.25 mm, and the case wherethe inclination angle θ is 1.0° corresponds to the case where the shiftof the light beam from the optical axis at the objective lens 206 is0.50 mm. Accordingly, even if the detector shift indicated by the dottedline in FIG. 14(a) is +20 μm, for example, it may be seen that only anextremely small focal offset of approximately 0.8 μm is generated. Inother words, the focal offset is less than one-half the focal offset ofthe conventional case.

FIG. 15 is a diagram showing the relationship of the detector shift andthe focal offset in the prior art based on the simulation results ofFIGS. 13(a) through 13(d). In FIG. 15, black circular marks indicateexperimental data. In FIG. 15, a coarse dotted line shows a case wherethe mounting error of the composite prism 215 is 5%, a fine dotted lineindicates a case where the mounting error of the composite prism 215 is10%, a two-dot chain line indicates a case where the shift of the lightbeam from the optical axis at the objective lens 206 is 0.25 mm, and aone-dot chain line indicates a case where the shift of the light beamfrom the optical axis at the objective lens 206 is 0.50 mm. As may beseen from FIG. 15, the focal offset is generated in each case where thedetector shift occurs.

On the other hand, FIG. 16 is a diagram showing the relationship of thedetector shift and the focal offset in the first or third embodimentbased on the simulation results of FIGS. 14(a) through 14(d). In FIG.16, black circular marks indicate experimental data. In FIG. 16, acoarse dotted line shows a case where the mounting error of thecomposite prism 15 or 15A is 5%, a fine dotted line indicates a casewhere the mounting error of the composite prism 15 or 15A is 10%, atwo-dot chain line indicates a case where the shift of the light beamfrom the optical axis at the objective lens 206 is 0.25 mm, and aone-dot chain line indicates a case where the shift of the light beamfrom the optical axis at the objective lens 206 is 0.50 mm. As may beseen from FIG. 16, the focal offset which is generated is extremelysmall or approximately 0 in each case where the detector shift occurs.Accordingly, it can be seen that the focal offset in the first or thirdembodiment is extremely small compared to that of the prior art.

In FIG. 1, the arrangement of the 4-part photodetector 216 along theoptical axis must be set approximately to the image formation pointposition of the condenser lens 212, due to the operating principle ofthe Foucault technique. On the other hand, the arrangement of the 2-partphotodetector 214 along the optical axis must be set at a positionshifted from the image formation point position of the condenser lens212, due to the operating principle of the push-pull technique. In otherwords, the 2-part photodetector 214 must be set at the so-called farfield.

For the above reasons, it is necessary to split the reflected light beaminto two by use of the beam splitter 213, and independently provide anoptical path which is used to carry out the Foucault technique and anoptical path which is used to carry out the push-pull technique. As aresult, if the focal error is to be detected using the Foucaulttechnique and the tracking error is to be detected using the push-pulltechnique, the optical system occupies a relatively large space becauseof the need to provide two independent optical paths, and furthermore,the number of parts required becomes large.

Accordingly, a description will hereinafter be given of embodiments ofthe optical information recording/reproducing apparatus according to thepresent invention which reduce the space of the optical system occupyingwithin the optical information recording/reproducing apparatus andreduce the number of required parts, so that the size and cost of theoptical information recording/reproducing apparatus and the optical diskunit using the same can both be reduced.

First, a description will be given of a fifth embodiment of the opticalinformation recording/reproducing apparatus according to the presentinvention, by referring to FIGS. 17 through 19. In FIG. 17, those partswhich are the same as those corresponding parts in FIG. 1 are designatedby the same reference numerals, and a description thereof will beomitted.

In this embodiment, it is unnecessary to provide the beam splitter 213and the 2-part photodetector 214 shown in FIG. 1, as may be seen fromFIG. 17. In addition, a composite prism 35 and a photodetector unit 36are provided in place of the composite prism 215 and the 4-partphotodetector 216. In other words, this embodiment uses the central partof the reflected light beam which is not used in the first throughfourth embodiments, for detecting the tracking error by the push-pulltechnique.

FIGS. 18(a) and 18(b) show the composite prism 35 on an enlarged scale.FIG. 18(a) shows a perspective view of the composite prism 35, and FIG.18(b) shows a plan view of the composite prism 35. As shown in FIGS.18(a) and 18(b), the composite prism 35 includes tapered first andsecond parts 35 a and 35 b, and a third part 35 c which has a convexsurface with a slight curvature. Hence, a reflected light beam 30 whichis obtained via the beam splitter 208 is split into three light beams 30a, 30 b and 30 c.

FIG. 19 is a perspective view, on an enlarged scale, showing animportant part of FIG. 17. The photodetector unit 36 includes a firstphotodetector 36 a, a second photodetector 36 b, and a thirdphotodetector 36 c. The first photodetector 36 a includes photodetectors37 a and 37 b. The second photodetector 36 b includes photodetectors 37c and 37 d. The third photodetector 36 c includes photodetectors 37 eand 37 f.

Out of the reflected light beam 30 which is refracted and condensed viathe condenser lens 212, the light beam 30 a which is transmitted throughthe first part 35 a is deflected depending on the taper angle of thefirst part 35 a and is irradiated on the first photodetector 36 a of thephotodetector unit 36, while the light beam 30 b which is transmittedthrough the second part 35 b is deflected depending on the taper angleof the second part 35 b and is irradiated on the second photodetector 36b of the photodetector unit 36. In addition, the light beam 30 c whichis transmitted through the third part 35 c is refracted depending on thecurvature of the third part 35 c and is irradiated on the thirdphotodetector 36 c of the photodetector unit 36. In other words, thelight beams 30 a and 30 b are only subjected to the refraction functionof the condenser lens 212, but the light beam 30 c is subjected to therefraction function of the condenser lens 212 and the third part 35 citself. Therefore, image formation points 300 a and 300 b of therespective light beams 30 a and 30 b are different from an imageformation point 300 c of the light beam 30 c. That is, distances L1 andL2 from the condenser lens 212 to the image formation points 300 a and300 b of the respective light beams 30 a and 30 b are different from adistance L3 from the condenser lens 212 to the image formation point 300c of the light beam 30 c.

In FIG. 19, the photodetector unit 36 is arranged on a plane which isperpendicular to the optical axis of the reflected light beam 30 andincludes the image formation points 300 a and 300 b. Because of thisarrangement, the first and second photodetectors 36 a and 36 b which areused to generate the focal error signal FES based on the Foucaulttechnique are respectively provided at the positions of the imageformation points 300 a and 300 b of the light beams 30 a and 30 b. Onthe other hand, the third photodetector 36 c which is used to generatethe tracking error signal TES based on the push-pull technique isprovided at a position deviated from the position of the image formationpoint 300 c of the light beam 30 c. Hence, it is possible to generatethe focal error signal FES using the Foucault technique and to generatethe tracking error signal TES using the push-pull technique by use of asimple optical system. The generation itself of the focal error signalFES and the tracking error signal TES may be made similarly to the priorart, and a description thereof will be omitted.

The requirement is that the distances L1 and L2 between the condenserlens 212 and the respective image formation points 300 a and 300 b ofthe light beams 30 a and 30 b are different from the distance L3 betweenthe condenser lens 212 and the image formation point 300 c of the lightbeam 30 c, and the construction and arrangement of the composite prism35 and the photodetector unit 36 are not limited to those of the aboveembodiment.

Next, a description will be given of a sixth embodiment of the opticalinformation recording/reproducing apparatus according to the presentinvention, by referring to FIGS. 20 and 21. In FIGS. 20 and 21, thoseparts which are the same as those corresponding parts in FIGS. 18 and 19are designated by the same reference numerals, and a description thereofwill be omitted.

In this embodiment, a composite prism 45 shown in FIGS. 20(a) and 20(b)is used in place of the composite prism 35 shown in FIG. 18.

FIGS. 20(a) and 20(b) show the composite prism 45 on an enlarged scale.FIG. 20(a) shows a perspective view of the composite prism 45, and FIG.20(b) shows a plan view of the composite prism 45. As shown in FIGS.20(a) and 20(b), the composite prism 45 includes tapered first andsecond parts 45 a and 45 b, and a third part 45 c which has a concavesurface with a slight curvature. Hence, a reflected light beam 30 whichis obtained via the beam splitter 208 is split into three light beams 30a, 30 b and 30 c.

FIG. 21 is a perspective view, on an enlarged scale, showing animportant part of this embodiment. The photodetector unit 36 is the sameas the photodetector unit 36 used in the fifth embodiment.

Out of the reflected light beam 30 which is refracted and condensed viathe condenser lens 212, the light beam 30 a which is transmitted throughthe first part 45 a is deflected depending on the taper angle of thefirst part 45 a and is irradiated on the first photodetector 36 a of thephotodetector unit 36, while the light beam 30 b which is transmittedthrough the second part 45 b is deflected depending on the taper angleof the second part 45 b and is irradiated on the second photodetector 36b of the photodetector unit 36. In addition, the light beam 30 c whichis transmitted through the third part 45 c is refracted depending on thecurvature of the third part 45 c and is irradiated on the thirdphotodetector 36 c of the photodetector unit 36. In other words, thelight beams 30 a and 30 b are only subjected to the refraction functionof the condenser lens 212, but the light beam 30 c is subjected to therefraction function of the condenser lens 212 and the third part 45 citself. Therefore, image formation points 300 a and 300 b of therespective light beams 30 a and 30 b are different from an imageformation point 300 c of the light beam 30 c. That is, distances L1 andL2 from the condenser lens 212 to the image formation points 300 a and300 b of the respective light beams 30 a and 30 b are different from adistance L3 from the condenser lens 212 to the image formation point 300c of the light beam 30 c.

In other words, the image formation point 300 c of the light beam 30 cis located between the composite prism 35 and the photodetector unit 36in the fifth embodiment, but the image formation point 300 c of thelight beam 30 c in this embodiment is located beyond the photodetectorunit 36 in FIG. 21 along the traveling direction of the light beam.

In FIG. 21, the photodetector unit 36 is arranged on a plane which isperpendicular to the optical axis of the reflected light beam 30 andincludes the image formation points 300 a and 300 b, similarly to thefifth embodiment shown in FIG. 19. Because of this arrangement, thefirst and second photodetectors 36 a and 36 b which are used to generatethe focal error signal FES based on the Foucault technique arerespectively provided at the positions of the image formation points 300a and 300 b of the light beams 30 a and 30 b. On the other hand, thethird photodetector 36 c which is used to generate the tracking errorsignal TES based on the push-pull technique is provided at a positiondeviated from the position of the image formation point 300 c of thelight beam 30 c. Hence, it is possible to generate the focal errorsignal FES using the Foucault technique and to generate the trackingerror signal TES using the push-pull technique by use of a simpleoptical system.

Next, a description will be given of a seventh embodiment of the opticalinformation recording/reproducing apparatus according to the presentinvention, by referring to FIGS. 22 and 23. In FIGS. 22 and 23, thoseparts which are the same as those corresponding parts in FIGS. 18 and 19are designated by the same reference numerals, and a description thereofwill be omitted.

In this embodiment, a composite prism 55 and a photodetector unit 56shown in FIG. 23 are used in place of the composite prism 35 and thephotodetector unit 36 shown in FIGS. 18(a) and 18(b).

FIGS. 22(a) and 22(b) show the composite prism 55 on an enlarged scale.FIG. 22(a) shows a perspective view of the composite prism 55, and FIG.22(b) shows a plan view of the composite prism 55. As shown in FIGS.22(a) and 22(b), the composite prism 55 includes tapered first andsecond parts 55 a and 55 b, and a flat third part 55 c which has nottaper. Hence, a reflected light beam 30 which is obtained via the beamsplitter 208 is split into three light beams 30 a, 30 b and 30 c.

FIG. 23 is a perspective view, on an enlarged scale, showing animportant part of this embodiment. The photodetector unit 56 includes afirst photodetector 56 a, a second photodetector 56 b, and a thirdphotodetector 56 c. The first photodetector 56 a includes photodetectors37 a and 37 b. The second photodetector 56 b includes photodetectors 37c and 37 d. The third photodetector 56 c includes photodetectors 37 eand 37 f. The third photodetector 56 c is arranged on a plane differentfrom a plane on which the first and second photodetectors 56 a and 56 bare arranged.

Out of the reflected light beam 30 which is refracted and condensed viathe condenser lens 212, the light beam 30 a which is transmitted throughthe first part 55 a is deflected depending on the taper angle of thefirst part 55 a and is irradiated on the first photodetector 56 a of thephotodetector unit 56, while the light beam 30 b which is transmittedthrough the second part 55 b is deflected depending on the taper angleof the second part 55 b and is irradiated on the second photodetector 56b of the photodetector unit 56. In addition, the light beam 30 c whichis transmitted through the third part 55 c is transmitted as it is andis irradiated on the third photodetector 56 c of the photodetector unit56. In other words, all of the light beams 30 a, 30 b and 30 c are onlysubjected to the refraction function of the condenser lens 212.Therefore, image formation points 300 a, 300 b and 300 c of therespective light beams 30 a, 30 b and 30 c are all located on the sameplane. That is, distances L1, L2 and L3 from the condenser lens 212 tothe image formation points 300 a, 300 b and 300 c of the respectivelight beams 30 a, 30 b and 30 c are the same. However, since the thirdphotodetector 56 c in this embodiment is arranged on the plane which isdifferent from the plane on which the first and second photodetectors 56a and 56 b are arranged, the image formation point 300 c of the lightbeam 30 c and the position of the third photodetector 56 c do not match.

In other words, the image formation point 300 c of the light beam 30 cis located between the composite prism 35 and the photodetector unit 36in the fifth embodiment, but the image formation point 300 c of thelight beam 30 c in this embodiment is located beyond the thirdphotodetector 56 c in FIG. 23 along the traveling direction of the lightbeam.

In FIG. 23, the first and second photodetectors 56 a and 56 b of thephotodetector unit 56 are arranged on a plane which is perpendicular tothe optical axis of the reflected light beam 30 and includes the imageformation points 300 a and 300 b, similarly to the fifth embodimentshown in FIG. 19. Because of this arrangement, the first and secondphotodetectors 56 a and 56 b which are used to generate the focal errorsignal FES based on the Foucault technique are respectively provided atthe positions of the image formation points 300 a and 300 b of the lightbeams 30 a and 30 b. On the other hand, the third photodetector 56 cwhich is used to generate the tracking error signal TES based on thepush-pull technique is provided at a position deviated from the positionof the image formation point 300 c of the light beam 30 c. Hence, it ispossible to generate the focal error signal FES using the Foucaulttechnique and to generate the tracking error signal TES using thepush-pull technique by use of a simple optical system.

The generation of the focal error signal FES based on the Foucaulttechnique is not limited to that of the embodiment using two lightbeams, and it is of course possible to use more than two light beams forthe generation of the focal error signal FES. Similarly, the generationof the tracking error signal TES based on the push-pull technique is notlimited to that of the embodiment using one light beam, and it is ofcourse possible to use more than one light beam for the generation ofthe tracking error signal TES.

Next, a description will be given of an eighth embodiment of the opticalinformation recording/reproducing apparatus according to the presentinvention, by referring to FIGS. 24, 25 and 26. In FIGS. 24 and 25,those parts which are the same as those corresponding parts in FIG. 17are designated by the same reference numerals, and a description thereofwill be omitted.

In this embodiment, an analyzer 208 A shown in FIGS. 24 and 25 is usedtogether with the composite prism 35 and the photodetector unit 36 shownin FIG. 17.

For example, an analyzer 21 disclosed in a Japanese Laid-Open PatentApplication No. 63-127436 may be used as the analyzer 208A. In thiseighth embodiment, the light beam is split into three light beams by theanalyzer 208A, and each of the three light beams are further split intothree light beams by the composite prism 35, thereby resulting in nine(3×3=9) light beams being output from the composite prism 35. The ninelight beams from the composite prism 35 are irradiated on correspondingones of nine photodetectors 66 a through 66 i which form thephotodetector unit 66.

FIG. 26 shows a plan view of the photodetector unit 66. The focal errorsignal FES can be generated according to the Foucault technique based onoutputs of the photodetectors 66 a, 66 b, 66 d, 66 e, 66 g and 66 h ofthe photodetector unit 66. The photodetectors 66 a, 66 d and 66 greceive the three light beams from the first part of the composite prism35, while the photodetectors 66 b, 66 e and 66 h receive the three lightbeams from the second part of the composite prism 35. The imageformation points of these six light beams match the positions of thephotodetectors 66 a, 66 b, 66 d, 66 e, 66 g and 66 h. On the other hand,the tracking error signal TES can be generated according to thepush-pull technique based on outputs of the photodetectors 66 c, 66 fand 66 i. The photodetectors 66 c, 66 f and 66 i receive the three lightbeams from the third part of the composite prism 35. The image formationpoints of these three light beams are deviated from the positions of thephotodetectors 66 c, 66 f and 66 i.

As shown in FIG. 26, the photodetector 66 a includes photodetector parts37 a and 37 b, the photodetector 66 b includes photodetector parts 37 cand 37 d, . . . , and the photodetector 66 i includes photodetectorparts 37 q and 37 r. Accordingly, if the outputs of these photodetectorparts 37 a through 37 i are denoted by the same reference numerals asthese parts, the focal error signal FES using the Foucault technique canbe generated based on any one of the following formulas (3a) through(3d) by calculation. $\begin{matrix}\begin{matrix}{{FES} = \quad {\left\lbrack {\left( {37a} \right) + \left( {37g} \right) + \left( {37m} \right) + \left( {37d} \right) + \left( {37j} \right) + \left( {37p} \right)} \right\rbrack -}} \\{\quad \left\lbrack {\left( {37b} \right) + \left( {37h} \right) + \left( {37n} \right) + \left( {37c} \right) + \left( {37i} \right) + \left( {37o} \right)} \right\rbrack}\end{matrix} & \text{(3a)} \\{{FES} = {\left\lbrack {\left( {37a} \right) + \left( {37p} \right)} \right\rbrack - \left\lbrack {\left( {37b} \right) + \left( {37o} \right)} \right\rbrack}} & \text{(3b)} \\{{FES} = {\left\lbrack {\left( {37m} \right) + \left( {37d} \right)} \right\rbrack - \left\lbrack {\left( {37n} \right) + \left( {37c} \right)} \right\rbrack}} & \text{(3c)} \\\begin{matrix}{{FES} = \quad {\left\lbrack {\left( {37a} \right) + \left( {37d} \right) + \left( {37m} \right) + \left( {37p} \right)} \right\rbrack -}} \\{\quad \left\lbrack {\left( {37b} \right) + \left( {37c} \right) + \left( {37n} \right) + \left( {37o} \right)} \right\rbrack}\end{matrix} & \text{(3d)}\end{matrix}$

The focal error signal FES obtained by the formula (3 a) has a highsignal-to-noise (S/N) ratio and is uneasily affected by externaldisturbances because all of the beams irradiated on the photodetectorunit 66 is used and consequently a large amount of light is used todetect the focal error signal FES.

On the other hand, the focal error signal FES obtained by the formula(3b) or (3c) enables easy and simple adjustments since only two beamsare used to detect the focal error signal FES and thus only two beamsneed to be irradiated on corresponding two photodetectors 66 a and 66 hor, 66 b and 66 g, of the photodetector unit 66 so that each beamirradiates a division line separating the two photodetector partsforming each of the photodetectors 66 a and 66 h or, 66 b and 66 g.

Furthermore, the focal error signal FES obtained by the formula (3d) canobtain, to a certain extent, the above described effects obtainable withrespect to the focal error signals FES obtained by the formulas (3a) and(3b) or (3c). In addition, in the case of the focal error signal FESobtained by the formula (3d), the required adjustments are simplercompared to the case of the focal error signal FES obtained by theformula (3a). Moreover, the focal error signal FES obtained by theformula (3d) has a higher S/N ratio and is less affected by externaldisturbances as compared to the focal error signal FES obtained by theformulas (3b) or (3c).

The focal error signal FES according to the above described formulas(3a) through (3d) can be generated by use of known adders andsubtracter.

The tracking error signal TES using the push-pull technique can begenerated based on one of the following formulas (4a) and (4b) bycalculation. $\begin{matrix}{{TES} = {\left\lbrack {\left( {37e} \right) + \left( {37k} \right) + \left( {37q} \right)} \right\rbrack - \left\lbrack {\left( {37f} \right) + \left( {37l} \right) + \left( {37r} \right)} \right\rbrack}} & \text{(4a)} \\{{TES} = {\left\lbrack {\left( {37e} \right) + \left( {37q} \right)} \right\rbrack - \left\lbrack {\left( {37f} \right) + \left( {37r} \right)} \right\rbrack}} & \text{(4b)}\end{matrix}$

The tracking error signal TES obtained by the formula (4a) has a highS/N ratio and is uneasily affected by external disturbances because allof the beams irradiated on the photodetector unit 66 is used andconsequently a large amount of light is used to detect the trackingerror signal TES.

On the other hand, the tracking error signal TES obtained by the formula(4b) enables easy and simple adjustments since only two beams are usedto detect the tracking error signal TES and thus only two beams need tobe irradiated on corresponding two photodetectors 66 c and 66 i of thephotodetector unit 66 so that each beam irradiates a division lineseparating the two photodetector parts forming each of thephotodetectors 66 c and 66 i.

Furthermore, by the function of the analyzer 208A, a magneto-opticsignal (information signal) MO which is recorded on the disk 207 can bereproduced based on one of the following formulas (5a) and (5b) bycalculation.

The tracking error signal TES according to the above described formulas(4a) and (4b) can be generated by use of known adders and subtracter.$\begin{matrix}\begin{matrix}{{MO} = \quad {\left\lbrack {\left( {37a} \right) + \left( {37b} \right) + \left( {37e} \right) + \left( {37f} \right) + \left( {37c} \right) + \left( {37d} \right)} \right\rbrack -}} \\{\quad \left\lbrack {\left( {37m} \right) + \left( {37n} \right) + \left( {37q} \right) + \left( {37r} \right) + \left( {37o} \right) + \left( {37p} \right)} \right\rbrack}\end{matrix} & \text{(5a)} \\\begin{matrix}{{MO} = \quad {\left\lbrack {\left( {37a} \right) + \left( {37b} \right) + \left( {37c} \right) + \left( {37d} \right)} \right\rbrack -}} \\{\quad \left\lbrack {\left( {37m} \right) + \left( {37n} \right) + \left( {37o} \right) + \left( {37p} \right)} \right\rbrack}\end{matrix} & \text{(5b)}\end{matrix}$

According to the magneto-optic signal MO obtained by the formula (5a),it is possible to obtain an average signal amplitude which is relativelylarge. On the other hand, according to the magneto-optic signal MOobtained by the formula (5b), it is possible to obtain a relatively highresolution.

The magneto-optic signal MO according to the above described formulas(5a) and (5b) can be generated by use of known adders and subtracter.

The disk 207 is also recorded with positional information whichindicates a track, sector and the like on the disk 207. This positionalinformation is often referred to as an identification signal ID, andthis identification signal ID can be recorded in the form of pits on thedisk 207 as is well known. The identification signal ID can bereproduced based on the following formula (6) by calculation. In otherwords, the magneto-optic signal MO is reproduced by obtaining adifference between the outputs of the adders, while the identificationsignal ID is reproduced by obtaining a sum of the outputs of the adders.$\begin{matrix}\begin{matrix}{{ID} = \quad {\left\lbrack {\left( {37a} \right) + \left( {37b} \right) + \left( {37e} \right) + \left( {37f} \right) + \left( {37c} \right) + \left( {37d} \right)} \right\rbrack +}} \\{\quad \left\lbrack {\left( {37m} \right) + \left( {37n} \right) + \left( {37q} \right) + \left( {37r} \right) + \left( {37o} \right) + \left( {37p} \right)} \right\rbrack}\end{matrix} & \text{(6)}\end{matrix}$

The identification signal ID according to the above described formula(6) can be generated by use of known adders.

According to the eighth embodiment, the magneto-optic signal detectionsystem and the servo signal detection signal can be providedapproximately on a single optical path, and it is therefore possible tofurther reduce both the size and cost of the optical informationrecording/reproducing apparatus compared to the fifth through seventhembodiments. As is evident from a comparison of FIGS. 17 and 24, theWollaston prism 209, the lens 210 and the 2-part photodetector 211required in FIG. 17 are omitted in FIG. 24.

Next, a description will be given of a first modification of the eighthembodiment of the optical information recording/reproducing apparatusaccording to the present invention, by referring to FIGS. 27 and 28. InFIGS. 27 and 28, those parts which are the same as those correspondingparts in FIGS. 25 and 26 are designated by the same reference numerals,and a description thereof will be omitted.

In this first modification of the eighth embodiment, a photodetectorunit 66A shown in FIG. 27 is used in place of the photodetector unit 66shown in FIG. 25.

FIG. 28 shows a plan view of the photodetector unit 66A. As shown inFIG. 28, the photodetector unit 66A includes 2-part photodetectors66A-1, 66A-2 and 66A-3, and photodetectors 66A-4 and 66A-5. The 2-partphotodetector 66A-1 includes photodetector parts A and B. The 2-partphotodetector 66A-2 includes photodetector parts C and D. The 2-partphotodetector 66A-3 includes photodetector parts E and F. Thephotodetector 66A-4 includes a photodetector part G, and thephotodetector 66A-5 includes a photodetector part H.

Accordingly, if the outputs of these photodetector parts A through H aredenoted by the same reference characters as these parts, the focal errorsignal FES using the Foucault technique can be generated based on thefollowing formula (7) by calculation.

FES=(A+C)−(B+D)   (7)

The focal error signal FES according to the above described formula (7)can be generated by use of known adders and subtracter.

In addition, the tracking error signal TES using the push-pull techniquecan be generated based on the following formula (8) by calculation.

TES=E−F   (8)

The tracking error signal TES according to the above described formula(8) can be generated by use of a known subtracter.

Furthermore, by the function of the analyzer 208A, the magneto-opticsignal MO which is recorded on the disk 207 can be reproduced based onthe following formula (9) by calculation.

MO=G−H   (9)

The magneto-optic signal MO according to the above described formula (9)can be generated by use of a known subtracter.

Moreover, the identification signal ID which is recorded on the disk 207can be reproduced based on the following formula (10) by calculation.

ID=G+H   (10)

The identification signal ID according to the above described formula(10) can be generated by use of a known adder.

As may be seen from FIG. 28 and the formulas (7) through (10) describedabove, this first modification of the eighth embodiment can obtain thefocal error signal FES, the tracking error signal TES, the magneto-opticsignal MO and the identification signal ID by use of a smaller number ofphotodetectors (or photodetector parts) and a smaller number of addersas compared to the eighth embodiment. Hence, due to the smaller numberof photodetectors (or photodetector parts) used, a stray capacitance ofthe photodetectors can be reduced compared to the eighth embodiment. Inaddition, due to the smaller number of adders used, a noise generated bythe adders within a large scale integrated circuit (LSI) can be reducedcompared to the eighth embodiment.

Compared to servo signals such as the focal error signal FES and thetracking error signal TES, the magneto-optic signal MO and theidentification signal ID require higher frequency bands, andconsequently, higher signal-to-noise (S/N) ratios. The stray capacitanceand noise described above affect the S/N ratios of the magneto-opticsignal MO and the identification signal ID, however, this firstmodification of the eighth embodiment can further improve the S/N ratiosof the magneto-optic signal MO and the identification signal ID comparedto the eighth embodiment due to the smaller number of photodetectors (orphotodetector parts) and the smaller number of adders used to reproducethese signals MO and ID.

Next, a description will be given of a second modification of the eighthembodiment of the optical information recording/reproducing apparatusaccording to the present invention, by referring to FIGS. 29 through 31.In FIG. 29, those parts which are the same as those corresponding partsin FIG. 25 are designated by the same reference numerals, and adescription thereof will be omitted.

In this second modification of the eighth embodiment, the analyzer 208Ais used together with a composite prism 35B shown in FIG. 30 and aphotodetector unit 66B shown in FIG. 29.

The reflected light beam from the disk 207 is split into three lightbeams by the analyzer 208A, and each of the three light beams arefurther split into five light beams by the composite prism 35B, therebyresulting in fifteen (3×5=15) light beams being output from thecomposite prism 35B. The fifteen light beams from the composite prism35B are irradiated on corresponding ones of nine photodetectors 66 aathrough 66 ii which form the photodetector unit 66B.

FIG. 30 shows a perspective view of the composite prism 35B. As shown inFIG. 30, the composite prism 35B includes tapered first and second parts35B-1 and 35B-2, a central third part 35B-3 which has a convex surfacewith a slight curvature, and peripheral fourth and fifth parts 35B-4 and35B-5 which have convex surfaces with a slight curvature matching thatof the third part 35B-3. In other words, the third, fourth and fifthparts 35B-3, 35B-4 and 35B-5 are all parts of a single convex surfacehaving a slight curvature. The first and second parts 35B-1 and 35B-2function similarly to the first and second parts 35 a and 35 b of thecomposite prism 35.

FIG. 31 shows a plan view of the photodetector unit 66B. As shown inFIG. 31, the photodetector unit 66B includes photodetectors 66 aathrough 66 ii. The photodetector 66 aa includes photodetector parts 37aa and 37 bb, the photodetector 66 bb includes photodetector parts 37 ccand 37 dd, . . . , and the photodetector 66 ii includes photodetectorparts 37 qq and 37 rr.

The three light beams output from the third part 35B-3 of the compositeprism 35B are respectively irradiated on the photodetectors 66 cc, 66 ffand 66 ii. The three light beams output from the fourth part 35B-4 ofthe composite prism 35B are respectively irradiated on thephotodetectors 66 cc, 66 ff and 66 ii. Further, the three light beamsoutput from the fifth part 35B-5 of the composite prism 35B arerespectively irradiated on the photodetectors 66 cc, 66 ff and 66 ii. Onthe other hand, the three light beams output from the first part 35B-1of the composite prism 35B are respectively irradiated on thephotodetectors 66 aa, 66 dd and 66 gg. The three light beams output fromthe second part 35B-2 of the composite prism 35B are respectivelyirradiated on the photodetectors 66 bb, 66 ee and 66 hh.

The image formation points of the six light beams output from the firstand second parts 35B-1 and 35B-2 match the positions of thephotodetectors 66 aa, 66 bb, 66 dd, 66 ee, 66 gg and 66 hh. On the otherhand, the image formation points of the three light beams output fromeach of the third, fourth and fifth parts 35B-3, 35B-4 and 35B-5 aredeviated from the positions of the photodetectors 66 cc, 66 ff and 66ii.

If the outputs of the photodetector parts 37 aa through 37 ii of thephotodetector unit 66B are denoted by the same reference numerals asthese parts, the focal error signal FES using the Foucault technique,the tracking error signal TES using the push-pull technique and theidentification signal ID can be obtained by calculations similarly tothe eighth embodiment described above based on the formulas (3a) through(3d), (4a), (4b) and (6).

Furthermore, the magneto-optic signal MO recorded on the disk 207 can bereproduced based on one of the following formulas (11a) and (11b) bycalculation. $\begin{matrix}\begin{matrix}{{MO} = \quad {\left\lbrack {\left( {37{aa}} \right) + \left( {37{bb}} \right) + \left( {37{ee}} \right) + \left( {37{ff}} \right) + \left( {37{cc}} \right) + \left( {37{dd}} \right)} \right\rbrack -}} \\{\quad \left\lbrack {\left( {37m\quad m} \right) + \left( {37{nn}} \right) + \left( {37{qq}} \right) + \left( {37{rr}} \right) + \left( {37{oo}} \right) + \left( {37{pp}} \right)} \right\rbrack}\end{matrix} & \text{(11a)} \\{{MO} = {\left\lbrack {\left( {37{ee}} \right) + \left( {37{ff}} \right)} \right\rbrack - {\left\lbrack {\left( {37{qq}} \right) + \left( {37{rr}} \right)} \right\rbrack \text{~~~~~~~~~~~~~~~~~}}}} & \text{(11b)}\end{matrix}$

According to the magneto-optic signal MO obtained by the formula (11a),it is possible to obtain an average signal amplitude which is relativelylarge. On the other hand, according to the magneto-optic signal MOobtained by the formula (11b), it is possible to obtain a relativelyhigh resolution. Moreover, the resolution obtainable according to themagneto-optic signal MO obtained by the formula (11b) is furtherimproved compared to that obtainable according to the formula (5b). Thereason for this further improved resolution of the magneto-optic signalMO using the composite prism 35B having the shape shown in FIG. 30 maybe understood from the teachings of the Proceedings of Magneto-OpticalRecording International Symposium '96, J. Magn. Soc. Jpn., Vol.20,Supplement No.S1 (1996), pp.233-238.

The magneto-optic signal MO according to the above described formulas(11a) and (11b) can be generated by use of known adders and subtracter.

In this second modification of the eighth embodiment, it is of coursepossible to use the photodetector unit 66A shown in FIG. 28 in place ofthe photodetector unit 66B. In this case, the focal error signal FES,the tracking error signal TES, the magneto-optic signal MO and theidentification signal ID can be obtained by calculations similarly tothe first modification of the eighth embodiment described above based onthe formulas (7) through (10).

In the eighth embodiment and the first and second modifications thereof,it is of course possible to arrange the composite prism 35 or 35Bbetween the analyzer 208A and the condenser lens 212.

In the fifth through eighth embodiments and the modifications of theeighth embodiment, the image formation point of the light beam used togenerate the focal error signal FES according to the Foucault techniqueand the image formation point of the light beam used to generate thetracking error signal TES according to the push-pull technique are mademutually different by use of the composite prism. However, the method ofmaking the image formation points of the light beams mutually differentis not limited to that using the composite prism, and it is alsopossible to use a holographic optical element, for example.

Next, a description will be given of a ninth embodiment of the opticalinformation recording/reproducing apparatus according to the presentinvention, by referring to FIGS. 32 and 33. In FIGS. 32 and 33, thoseparts which are the same as those corresponding parts in FIG. 19 aredesignated by the same reference numerals, and a description thereofwill be omitted.

In this embodiment, a holographic optical element 75 shown in FIG. 32 isused in place of the composite prism 35 shown in FIG. 19.

FIG. 32 is a perspective view showing an important part of thisembodiment on an enlarged scale. The holographic optical element 75includes first and second parts 75 a and 75 b. The cross sectional shapeof the first part 75 a along a line A-A′ in FIG. 32 is a sawtoothgrating as shown in FIG. 33. The second part 75 b has a cross sectionalshape similar to that of the first part 75 a, but the cross sectionalshape of the second part 75 b is in point symmetry to that of the firstpart 75 a with respect to the center of the holographic optical element75. The sawtooth gratings of the first and second parts 75 a and 75 bare sometimes also referred to as blazed gratings.

The holographic optical element 75 separates the reflected light beam 30into 0th order diffracted light, ±1st order diffracted lights, andhigh-order diffracted lights of ±2nd order or higher. In thisembodiment, the cross sectional shape of the holographic optical element75 is designed so that the effect of high-order diffracted lights of±2nd order or higher are small when detecting the light. With regard tothe ±1st order diffracted lights, the above described sawtooth crosssectional shapes of the first and second parts 75 a and 75 b aredesigned so that, for example, the quantity of the emitted +1st orderdiffracted light is larger than that of the emitted −1st orderdiffracted light, that is, so that the effects of the −1st orderdiffracted light which is a divergent ray is minimized.

Accordingly, this embodiment uses a +1st order diffracted light 30-1which is diffracted by the grating of the first part 75 a, a +1st orderdiffracted light 30-2 which is diffracted by the grating of the secondpart 75 b, and a 0th order diffracted light 30-3 which passes throughthe first and second parts 75 a and 75 b without being affected by thegratings thereof. In addition, the grating patterns of the first andsecond parts 75 a and 75 b are designed such that the +1st orderdiffracted light 30-1 which is emitted from the first part 75 a isrefracted twice via the condenser lens 212 and the first part 75 abefore being imaged at an image formation point 300 a, and the +1storder diffracted light 30-2 which is emitted from the second part 75 bis refracted twice via the condenser lens 212 and the second part 75 bbefore being imaged at an image formation point 300 b. On the otherhand, since the 0th order diffracted light 30-3 passes through theholographic optical element 75 as it is without being affected by thegrating patterns, the 0th order diffracted light 30-3 is refracted onlyby the condenser lens 212 and is imaged at an image formation point 300c.

In FIG. 32, the photodetector unit 36 is arranged on a plane which isperpendicular to the optical axis of the reflected light beam andincludes the image formation points 300 a and 300 b. Because of thisarrangement, the first and second photodetectors 36 a and 36 b which areused to generate the focal error signal FES based on the Foucaulttechnique are respectively provided at the positions of the imageformation points 300 a and 300 b of the +1st order diffracted lights30-1 and 30-2. On the other hand, the third photodetector 36 c which isused to generate the tracking error signal TES based on the push-pulltechnique is provided at a position deviated from the position of theimage formation point 300 c of the 0th order diffracted light 30-3.Hence, it is possible to generate the focal error signal FES using theFoucault technique and to generate the tracking error signal TES usingthe push-pull technique by use of a simple optical system. Thegeneration itself of the focal error signal FES and the tracking errorsignal TES may be made similarly to the prior art, and a descriptionthereof will be omitted.

The requirement is that the distances L1 and L2 between the condenserlens 212 and the respective image formation points 300 a and 300 b ofthe +1st order diffracted lights 30-1 and 30-2 are different from thedistance L3 between the condenser lens 212 and the image formation point300 c of the 0th order diffracted light 30-3, and the construction andarrangement of the holographic optical element 75 and the photodetectorunit 36 are not limited to those of this embodiment.

Next, a description will be given of the functions of the holographicoptical element 75 by itself, that is, for the case where no condenserlens 212 exists, by referring to FIGS. 34 through 36.

As described above, the holographic optical element 75 includes thefirst and second parts 75 a and 75 b which are provided with independentpatterns for deflecting, converging and diverging the light. Moreparticularly, the patterns of the first and second parts 75 a and 75 bare respectively set so that the +1st order diffracted light 30-1 fromthe first part 75 a converges to a point P′(−x, 0 ) and the +1st orderdiffracted light 30-2 from the second part 75 b converges to a pointP(x, 0 ). Points P and P′ are located on a plane x which is a distance faway from the holographic optical element 75 along the optical axis. Inother words, the function of the first part 75 a is to image theparallel incident light at a focal point 0 at a focal distance f, and toconverge the light to the point P′ by deflecting the light by a distancex in the negative x-direction.

FIG. 35 shows a plan view of the holographic optical element 75. Sincethe patterns of the first and second parts 75 a and 75 b are in pointsymmetry with respect to the origin 0 in FIG. 35, the pattern of thefirst part 75 a, for example, is made up of concentric grooves orprojections having a center at the point P′(−x, 0). A radius r_(i) of anith concentric groove or projection can be obtained from the followingformula (12), where λ denotes the wavelength of the light output fromthe light source.

r _(i)={square root over ((2·f·λ·i+L )}  (12)

In addition, the cross sectional shape of the first part 75 a isdetermined so that the ratios of the 0th order diffracted light and the+1st order diffracted light with respect to the total quantity of lightbecome predetermined values.

In the above ninth embodiment, the cross sectional shape of theholographic optical element 75 is designed so that the effects of thehigh-order diffracted lights of ±2nd order diffracted lights or higherare small when detecting the light. In addition, with respect to the±1st order diffracted lights, the cross sectional shapes of the firstand second parts 75 a and 75 b of the holographic optical element 75 areset to the sawtooth shape shown in FIG. 33 so that the quantity of theemitted +1st order diffracted light is larger than that of the emitted−1st order diffracted light, that is, so that the effects of the −1storder diffracted light which is a divergent ray are minimized. However,it is of course possible to design the cross sectional shape of theholographic optical element 75 so that the quantity of the emitted −1storder diffracted light is larger than that of the emitted +1st orderdiffracted light, that is, so that the −1st order diffracted light whichis a divergent ray is positively used and the effects of the +1st orderdiffracted light are minimized.

In a tenth embodiment of the optical information recording/reproducingapparatus according to the present invention, the holographic opticalelement 75 used has a cross sectional shape shown in FIG. 36 along theline A-A′ in FIG. 32. An important part of this embodiment isessentially the same as FIG. 32, and an illustration thereof will beomitted. Contrary to the ninth embodiment, this embodiment positivelyuses the −1st order diffracted lights. For this reason, the first andsecond photodetectors 36 a and 36 b for generating the focal errorsignal FES based on the Foucault technique are provided at the imageformation points of the −1st order diffracted lights. On the other hand,the third photodetector 36 c for generating the tracking error signalTES based on the push-pull technique is provided at a position deviatedfrom the image formation point 300 c of the 0th order diffracted light,that is, between the holographic optical element 75 and thephotodetector unit 36. As a result, it is possible to generate the focalerror signal FES using the Foucault technique and to generate thetracking error signal TES using the push-pull technique by use of asimple optical system.

According to the structure shown in FIG. 34, the +1st order diffractedlight obtained from the first part 75 a of the holographic opticalelement 75 may overlap the −1st order diffracted light obtained from thesecond part 75 b, and the −1st order diffracted light obtained from thefirst part 75 a may overlap the +1st order diffracted light obtainedfrom the second part 75 b. For this reason, the holographic opticalelement 75 may be constructed so that by itself the holographic opticalelement 75 acts on the light as shown in FIG. 37. In FIG. 37, thoseparts which are the same as those corresponding parts in FIG. 34 aredesignated by the same reference numerals, and a description thereofwill be omitted.

In FIG. 37, the patterns of the first and second parts 75 a and 75 b areset so that the +1st order diffracted light 30-1 from the first part 75a converges to a point Q′(−x, y), the −1st order diffracted light fromthe first part 75 a is projected in a semicircular shape about a pointR′(x, −y), the +1st order diffracted light 30-2 from the second part 75b converges to a point Q(x, y), and the −1st order diffracted light isprojected in a semicircular shape about a point R(−x, −y). The points Q,Q′, R and R′ are located on the plane π which is the distance f from theholographic optical element 75 along the optical axis.

Next, a description will be given of an eleventh embodiment of theoptical information recording/reproducing apparatus according to thepresent invention, by referring to FIGS. 38 through 40. FIG. 38 showsthe eleventh embodiment, FIGS. 39(a) and 39(b) show a composite prism ofthe eleventh embodiment, and FIG. 40 shows a perspective view of animportant part of the eleventh embodiment. In FIG. 38, those parts whichare the same as those corresponding parts in FIG. 17 are designated bythe same reference numerals, and a description thereof will be omitted.

In this embodiment, the spot of the light beam irradiated on the disk207 via the objective lens 206 has a diameter of approximately 1 μm, forexample. In addition, the outputs of the 2-part photodetector 211 areused to generate an address signal ADR via an adder 311A, and theoutputs of the 2-part photodetector 211 are also used to reproduce themagneto-optic signal (information signal) MO via a differentialamplifier 311B.

In this embodiment, a composite prism 85 splits the reflected light beamwhich is obtained via the condenser lens 212 into first through fourthlight beams 87 a through 87 d. These first through fourth light beams 87a through 87 d are irradiated on a photodetector unit 86. Thephotodetector unit 86 includes a first photodetector 86 a which has 4light receiving parts A through D for receiving the first and secondlight beams 87 a and 87 b, a second photodetector 86 b which has a lightreceiving part E for receiving the third light beam 87 c, and a thirdphotodetector 86 c which has a light receiving part F for receiving thefourth light beam 87 d. As shown in FIG. 40, the first, second and thirdphotodetectors 86 a, 86 b and 86 c are arranged on the same plane. Thefirst through third photodetectors 86 a through 86 c may or may not beseparated from each other within the photodetector unit 86.

FIG. 39(a) shows a perspective view of the composite prism 85 on anenlarged scale, and FIG. 39(b) shows a plan view of the composite prism85. As shown, the composite prism 85 includes a first emission surface85 a for emitting the first light beam 87 a, a second emission surface85 b for emitting the second light beam 87 b, and third and fourthemission surfaces 85 c and 85 d for respectively emitting the third andfourth light beams 87 c and 87 d. In FIG. 39(a), the first emissionsurface 85 a has a downward inclination to the right, the secondemission surface 85 b has a downward inclination to the left, and thethird and fourth emission surfaces 85 c and 85 d form a mountain shape.In other words, the third emission surface 85 c has a downwardinclination to the right, the fourth emission surface 85 d has adownward inclination to the left, and the third and fourth emissionsurfaces 85 c and 85 d connect to form the mountain shape.

The first emission surface 85 a and the third emission surface 85 c areinclined towards the same direction, and an inclination angle α₁ of thefirst emission surface 85 a relative to a reference plane is smallerthan an inclination angle α₃ of the third emission surface 85 c. Forexample, the reference plane is the back surface of the composite prism85, which is approximately perpendicular to the traveling direction ofthe incoming reflected light beam. On the other hand, the secondemission surface 85 b and the fourth emission surface 85 d are inclinedtowards the same direction, and an inclination angle α₂ of the secondemission surface 85 b is smaller than an inclination angle α₄ of thefourth emission surface 85 d.

In FIG. 40, the first light beam 87 a which is emitted from the firstemission surface 85 a of the composite prism 85 is received by the lightreceiving parts A and D of the first photodetector 86 a. In addition,the second light beam 87 b which is emitted from the second emissionsurface 85 b of the composite prism 85 is received by the lightreceiving parts B and C of the first photodetector 86 a. Hence, a focalerror signal FES is generated according to the Foucault technique basedon the formula (2) described above. More particularly, the outputs ofthe light receiving parts A and C are added in an adder 321, the outputsof the light receiving parts B and D are added in an adder 322, and theoutputs of these adders 321 and 322 are supplied to a differentialamplifier 323 which outputs the focal error signal FES.

On the other hand, the third light beam 87 c which is emitted from thethird emission surface 85 c of the composite prism 85 is received by thelight receiving part E of the second photodetector 86 b, and the fourthlight beam 87 d which is emitted from the fourth emission surface 85 dof the composite prism 85 is received by the light receiving part F ofthe third photodetector 86 c. Hence, a tracking error signal TES isgenerated according to the push-pull technique based on the formula (1)described above. More particularly, the outputs of the light receivingparts E and F are supplied to a differential amplifier 331, and thetracking error signal TES is output from this differential amplifier331.

According to this embodiment, it is unnecessary to split the opticalpath into two by the beam splitter 213 shown in FIG. 1, even though theFoucault technique is used to generate the focal error signal FES andthe push-pull technique is used to generate the tracking error signalTES. For this reason, it is possible to reduce the space occupied by theoptical system within the optical information recording/reproducingapparatus. In addition, it is possible to reduce both the number ofparts and the cost of the optical information recording/reproducingapparatus because this embodiment does not require the beam splitter 213and the photodetector 214 shown in FIG. 1. Furthermore compared to thecase where the astigmatism technique is used to generate the focal errorsignal FES, it is possible to reduce the diameter of the beam spotformed on the photodetector and prevent effects of the externaldisturbance, thereby making it possible to improve the reliability ofthe optical information recording/reproducing apparatus.

Moreover, if the photodetector unit 86 is adjusted to detect apredetermined focal error signal. FES, it is possible to employ astructure that would automatically receive the third light beam 87 c bythe light receiving part E of the photodetector 86 b and receive thefourth light beam 87 d by the light receiving part F of thephotodetector 86 c. Hence, there is an additional advantage in that noadjustment is required in this case for the detection of the trackingerror signal TES.

Next, a description will be given of a twelfth embodiment of the opticalinformation recording/reproducing apparatus according to the presentinvention, by referring to FIGS. 41 through 44. FIG. 41 shows thetwelfth embodiment, and in FIG. 41, those parts which are the same asthose corresponding parts in FIG. 38 are designated by the samereference numerals, and a description thereof will be omitted.

In this embodiment, an integral part 90 is provided in place of thecomposite prism 85 shown in FIG. 38. In addition, the beam splitter 208,the Wollaston prism 209, the condenser lens 210 and the photodetector211 shown in FIG. 38 are not provided in FIG. 41.

The reflected light beam which is obtained via the beam splitter 204 isconverted by the condenser lens 212 and is input to the integral part 90which functions as a beam splitter means. Hence, the reflected lightbeam is split into first through sixth light beams 91 a through 91 f,and these first through sixth light beams 91 a through 91 f areirradiated on a photodetector unit 86A.

The integral part 90 integrally comprises a Wollaston prism 92 shown inFIG. 42 and the composite prism 85 shown in FIG. 39. In other words, theWollaston prism 92 is positioned immediately before the composite prism85 along the traveling direction of a reflected light beam 89, and isadhered on the back of the composite prism 85 as shown in FIGS. 43 and44.

The Wollaston prism 92 is made up of two triangular prisms 93 and 94which are cut from a crystal and adhered together. The size of theWollaston prism 92 corresponds to the central mountain shaped part ofthe composite prism 85. The Wollaston prism 92 is adhered on the back ofthe composite prism 85 immediately behind a mountain part 85 e of thecomposite prism 85. In addition, the Wollaston prism 92 extends for thefull width of the mounting part 85 e. Hence, the Wollaston prism 92splits the incoming reflected light beam in a direction in which avertex 85 f of the mountain part 85 e extends.

On the other hand, the photodetector unit 86A includes a firstphotodetector 86Aa, a second photodetector 86Ab-1. a third photodetector86Ab-2, a fourth photodetector 86Ac-1 and a fifth photodetector 86Ac-2which are provided on a single plane as shown in FIG. 44. The firstphotodetector 86Aa includes four light receiving parts A through D forreceiving the first and second light beams 91 a and 91 b. The secondphotodetector 86Ab-1 includes a light receiving part E₁ for receivingthe third light beam 91 c, and the third photodetector 86Ab-2 includes alight receiving part E₂ for receiving the fourth light beam 91 d. Thefourth photodetector 86Ac-1 includes a light receiving part F₁ forreceiving the fifth light beam 91 e, and the fifth photodetector 86Ac-2includes a light receiving part F₂ for receiving the sixth light beam 91f.

Out of the reflected light beam 89 which is input to the integral part90 via the condenser lens 212, a light component 89-1 which passes abovethe upper part of the Wollaston prism 92 in FIG. 43 and reaches thecomposite prism 85 directly is refracted by the first emission surface85 a and is emitted from the first emission surface 85 a as the firstlight beam 91 a. As shown in FIG. 44, this first light beam 91 a isreceived by the light receiving parts A and D of the first photodetector86Aa.

On the other hand, out of the reflected light beam 89 which is input tothe integral part 90 via the condenser lens 212, a light component 89-2which passes below the lower part of the Wollaston prism 92 in FIG. 43and reaches the composite prism 85 directly is refracted by the secondemission surface 85 b and is emitted from the second emission surface 85b as the second light beam 91 b. As shown in FIG. 44, this second lightbeam 91 b is received by the light receiving parts B and C of the firstphotodetector 86Aa.

A focal error signal FES is generated according to the Foucaulttechnique based on the formula (2) described above, similarly to theeleventh embodiment shown in FIG. 38.

Out of the reflected light beam 89 which is input to the integral part90 via the condenser lens 212, a light component 89-3 which reaches theWollaston prism 92 is split into a p-polarized light (wave) 95 and ans-polarized light (wave) 96. The p-polarized light 95 is deflected by anangle β with respect to an extension line 97 of the light component 89-3towards the first emission surface 85 a. On the other hand, thes-polarized light 96 is deflected by an angle β with respect to theextension line 97 towards the second emission surface 85 b.

The p-polarized light 95 and the s-polarized light 96 output from theWollaston prism 92 is input to the composite prism 85. The angle β issmall, and the p-polarized light 95 and the s-polarized light 96propagate within the mountain part 85 e of the composite prism 85. Thep-polarized light 95 and the s-polarized light 96 reach the third andfourth emission surfaces 85 c and 85 d and are refracted thereby, andare thereafter emitted from the third and fourth emission surfaces 85 cand 85 d.

In other words, in FIG. 44 the p-polarized light 95 is emitted from thethird emission surface 85 c as the third light beam 91 c. This thirdlight beam 91 c irradiates the light receiving part E₁ of the secondphotodetector 86Ab-1. On the other hand, the s-polarized light 96 isemitted from the third emission surface 85 c as-the fourth light beam 91d. This fourth light beam 91 d irradiates the light receiving part E₂ ofthe third photodetector 86Ab-2.

Similarly in FIG. 44, p-polarized light 95 is emitted from the fourthemission surface 85 d as the fifth light beam 91 e. This fifth lightbeam 91 e irradiates the light receiving part F₁ of the fourthphotodetector 86Ac-1. On the other hand, the s-polarized light 96 isemitted from the fourth emission surface 85 e as the sixth light beam 91f. This sixth light beam 91 f irradiates the light receiving part F₂ ofthe fifth photodetector 86Ac-2.

A tracking error signal TES is obtained based on the outputs of thelight receiving parts E₁, E₂, F₁ and F₂ of the second through fifthphotodetectors 86Ab-1 through 86Ac-2. More particularly, the trackingerror signal TES is obtained through adders 332 and 333 and thedifferential amplifier 331 shown in FIG. 44, by calculatingTES=(E₁+E₂)−(F₁+F₂).

In addition a magneto-optic signal (information signal) MO is obtainedbased on the outputs of the light receiving parts E₁, E₂, F₁ and F₂ ofthe second through fifth photodetectors 86Ab-1 through 86Ac-2. Moreparticularly, the magneto-optic signal MO is obtained through adders 312and 313, and the differential amplifier 311B shown in FIG. 44, bycalculating MO=(E₁+F₁)−(E₂+F₂).

Furthermore, an address signal ADR is obtained based on the outputs ofthe light receiving parts E₁, E₂, F₁ and F₂ of the second through fifthphotodetectors 86Ab-1 through 86Ac-2. More particularly, the addresssignal ADR is obtained through the adder 332, the adder 333, and theadder 311A shown in FIG. 44, by calculating ADR=(E₁+E₂)+(F₁+F₂).

According to this embodiment, it is possible to detect all of the focalerror signal FES, the tracking error signal TES, the magneto-opticsignal MO and the address signal ADR by use of a single optical path ofthe reflected light beam 89 and a single photodetector unit 86A.

By comparing FIG. 41 to FIG. 38, it may be seen that this twelfthembodiment shown in FIG. 4 i does not have the optical path whichextends horizontally from the beam splitter 208 in FIG. 38. For thisreason, the space occupied by the optical system within the opticalinformation recording/reproducing apparatus and the number of requiredparts are further reduced in this embodiment when compared to theeleventh embodiment. In other words, both the size and cost of theoptical information recording/reproducing apparatus in this embodimentcan further be reduced when compared to those of the eleventhembodiment.

In addition, if the photodetector unit 86A is adjusted to detect apredetermined focal error signal FES, it is possible to employ astructure that would automatically receive the third through sixth lightbeams 91 c through 91 f by the corresponding light receiving parts E₁,E₂, F₁ and F₂ of the second through fifth photodetectors 86Ab-1 through86Ac-2. Hence, there is an additional advantage in that no adjustment isrequired in this case for the detection of the tracking error signal TESand the magneto-optic signal MO.

Of course, independent Wollaston prism and composite prism may be usedin place of the integral part 90 which integrally comprises theWollaston prism 92 and the composite prism 85. In other words., theindependent Wollaston prism may be provided at a position to confrontthe back of the composite prism with a gap formed therebetween.

According to the fifth embodiment of the optical informationrecording/reproducing apparatus described above in conjunction withFIGS. 24, 25 and 26, the reflected light beam from the disk 207 is splitinto nine light beams which are irradiated on the photodetector unit 66.But the optical information recording/reproducing apparatus can be madeto have a more simple construction which is more compact, if the focalerror signal FES, the tracking error signal TES, the magneto-opticsignal MO and the identification signal ID can be obtained based ondetections of a smaller number of light beams at the photodetector unit.Hence, a description will now be given of further embodiments of theoptical information recording/reproducing apparatus which can obtain thesignals FES, TES, MO and ID based on a smaller number of light beamswithout greatly deteriorating the signal qualities.

FIG. 45 shows a thirteenth embodiment of the optical informationrecording/reproducing apparatus according to the present invention. Asshown in FIG. 45, a magneto-optic head unit of an optical informationrecording/reproducing apparatus is arranged under a magneto-optic disk505. A laser beam emitted from a laser diode 501 which is used as acoherent light source is converted into a parallel ray by a collimatorlens 502, and is input to a beam splitter 503. The parallel raytransmitted through the beam splitter 503 is converged on a magneticlayer of the magneto-optic disk 505 by an objective lens 504. Theconverged laser beam is reflected by the magneto-optic disk 505, and isinput again to the beam splitter 503. This time, the reflected lightbeam is reflected by the beam splitter 503 towards a photodetector unit509. More particularly, the reflected light beam is reflected by thebeam splitter 503 and is input to a Wollaston prism 506 which is ananalyzer.

FIG. 46 shows a perspective view of an important part of the thirteenthembodiment, that is, a signal detection system, on an enlarged scale.FIG. 46 shows a state where the light beam output from the Wollastonprism 506 is projected on the photodetector unit 509, viewed from theback surface side of the photodetector unit 509. The reflected lightbeam input to the Wollaston prism 506 is separated into two polarizedlight beams and input to a composite prism 507. As shown in FIG. 46, thecomposite prism 507 is an optical element having three deflectionsurfaces 571, 572 and 573 arranged side by side from the left to right.The deflection surfaces 571 and 573 on both sides of the composite prism507 have tapered surfaces for deflecting the light beam in upward anddownward directions by mutually different angles. On the other hand, thedeflection surface 572 at the central part of the composite prism 507has a convex shape having a curvature with respect to the optical axis.Each of the two polarized light beams input to the composite prism 507having the structure described above is spatially split into three bythe three deflection surfaces 571, 572 and 573, thereby outputting atotal of six light beams from the composite prism 507. The six lightbeams output from the composite prism 507 are projected on thephotodetector unit 509 via a condenser lens 508. For the sake ofconvenience, the illustration of the condenser lens 508 shown in FIG. 45is omitted in FIG. 46.

FIGS. 47 through 49 respectively show plan views of the photodetectorunit 509. FIG. 47 shows a state where the objective lens 504 and themagneto-optic disk 505 are close to each other. FIG. 48 shows a statewhere the laser beam is in focus on the magneto-optic disk 505.. Inaddition, FIG. 49 shows a state where the objective lens 504 and themagneto-optic disk 505 are far away from each other. For the sake ofconvenience, FIGS. 47 through 49 show the photodetector unit 509 viewedfrom a back surface side of the photodetector unit 509.

As may be seen from FIGS. 47 through 49, the photodetector unit 509 is alight receiving element, and photodetectors of the photodetector unit509 are arranged in three stages from the top to bottom in the verticaldirection. An upper stage portion of the photodetector unit 509 has aphotodetector including a photodetector part G for detecting amagneto-optic signal MO and an identification signal ID, and a 2-partphotodetector including photodetector parts A and B for detecting afocal error signal FES. A middle stage portion of the photodetector unit509 has a 2-part photodetector including photodetector parts E and F fordetecting a tracking error signal TES. Further, a lower stage portion ofthe photodetector unit 509 has a 2-part photodetector includingphotodetector parts C and D for detecting the focal error signal FES,and a photodetector including a photodetector part H for detecting themagneto-optic signal MO and the identification signal ID.

The 2-part photodetector in the lower stage portion and including thephotodetector parts C and D is arranged below the photodetector in theupper stage portion and including the photodetector part G for detectingthe magneto-optic signal MO and the identification signal ID. On theother hand, the 2-part photodetector in the upper stage portion andincluding the photodetector parts A and B is arranged above thephotodetector in the lower stage portion and including the photodetectorpart H for detecting the magneto-optic signal MO and the identificationsignal ID. The photodetector parts A and B are separated by a verticaldivision line, and the photodetector parts C and D are separated by avertical division line. The photodetector parts E and F are separated bya horizontal division line. In the following description, outputs of theabove described photodetector parts A through H are denoted by the samereference characters as these parts.

As shown in FIG. 47, the spots of the six light beams are formed on thephotodetector unit 509, and the outputs A through H are obtained fromthe corresponding photodetector parts A through H. The focal errorsignal FES using the Foucault technique can be generated based on thefollowing formula (13) by calculation, using known adders andsubtracter.

FES=(A+C)−(B+D)   (13)

The tracking error signal TES using the push-pull technique can begenerated based on the following formula (14) by calculation using aknown subtracter.

TES=E−F   (14)

The magneto-optic signal MO can be reproduced based on the followingformula (15) by calculation, using known adders and subtracter.

MO=(A+B+H)−(C+D+G).   (15)

In addition, the identification signal ID can be reproduced based on thefollowing formula (16a) or (16b) by calculation, using known adders.

ID=(A+B+H)+(C+D+G)   (16a)

ID=A+B+C+D+E+F+H+G   (16b)

When the identification signal ID is reproduced based on the formula(16a), the number of adders provided in the LSI is relatively small,thereby making it possible to improve the S/N ratio of theidentification signal ID. On the other hand, the identification signalID may become distorted depending on the shape and depth of the pitsformed in the magneto-optic disk 505. Hence, when the identificationsignal ID is reproduced based on the formula (16b), it is possible topositively prevent the identification signal ID from becoming distortedbecause all of the diffracted light beams from the pits of themagneto-optic disk 505 are used to detect the identification signal ID.

According to this embodiment, a beam splitter means comprises aplurality of beam splitting stages for splitting the reflected lightbeam from the magneto-optic disk 505 into a plurality of light beamswhich are projected onto a single light receiving element. Moreparticularly, the beam splitter means comprises the Wollaston prism 506and the composite prism 507 in this embodiment, and the reflected lightbeam from the magneto-optic disk 505 is split into six light beams. Thesix split light beams are projected onto the single photodetector unit509. As a result, the optical information recording/reproducingapparatus can be formed by a relatively small number of parts, and thearrangement of these relatively small number of parts can be simplyadjusted with ease. Furthermore, the cost of the optical informationrecording/reproducing apparatus can be reduced due to the relativelysmall number of parts used. In addition, since the photodetector unit509 is formed by only five photodetectors (eight photodetector parts Athrough H), it is possible to reduce the stray capacitance of thephotodetectors and to reduce the noise generated by the adders andsubtracter within the LSI which obtains the focal error signal FES, thetracking error signal TES, the magneto-optic signal MO and theidentification signal ID, as compared to the embodiment shown in FIGS.25 and 26, for example. Consequently, the signal qualities of thesesignals FES, TES, MO and ID can be improved.

In addition, the magneto-optic signal MO and the identification signalID are reproduced using also the outputs of the 2-part photodetectors(photodetector parts A, B, C and D) which detect the light beams forobtaining the focal error signal FES, in addition to using the outputsof the photodetectors (photodetector parts G and H) which detect thelight beams for obtaining the magneto-optic signal and theidentification signal ID. Thus, because of the small number ofphotodetector parts used to reproduce the magneto-optic signal MO andthe identification signal ID, the S/N ratio of these signal MO and ID isimproved. On the other hand, the required frequency bands of themagneto-optic signal MO and the identification signal ID are higher thanthose of the tracking error signal TES and the focal error signal FESwhich requires a frequency band lower than that of the tracking errorsignal TES. Further, when two or more kinds of signals are obtainedbased on the detection of the same light beam, the circuit constructionof the circuit used to obtain these signals becomes simpler as thefrequency bands of these signal become wider apart from each other.Therefore, by obtaining the magneto-optic signal MO and theidentification signal ID based on the detection of the same light beamthat is used to obtain the focal error signal FES, it is possible tosimplify the circuit construction also from this point of view.

Furthermore, because the magneto-optic signal MO and the identificationsignal ID are reproduced using also the outputs of the 2-partphotodetectors (photodetector parts A, B, C and D) which detect thelight beams for obtaining the focal error signal FES in addition tousing the outputs of the photodetectors (photodetector parts G and H)which detect the light beams for obtaining the magneto-optic signal andthe identification signal ID, the resolution of the magneto-optic signalMO and the identification signal ID is improved. It is taught in aJapanese Laid-Open Patent Application No. 7-6379 that the resolution ofthe magneto-optic signal is improved when a central portion of thereflected light beam from a recording medium is masked and themagneto-optic signal is detected from a peripheral portion of thereflected light beam. In this embodiment, the light beam used to detectthe focal error signal FES is the light beam transmitted through thetapered deflection surfaces 571 and 573 of the composite prism 507 asshown in FIG. 46. In other words, out of the reflected light beam fromthe magneto-optic disk 505, the light beam transmitted through thecentral deflection surface 572 of the composite prism 507 is not used,and only the light beam transmitted through the tapered deflectionsurfaces 571 and 573 on both sides of the central deflection surface 572of the composite prism 507 is used to detect the focal error signal FES.As a result, the resolution of the magneto-optic signal MO and theidentification signal ID is improved by obtaining the magneto-opticsignal MO and the identification signal ID using the light beam which istransmitted through the tapered deflection surfaces 571 and 573 of thecomposite prism 507 and is used to detect the focal error signal FES.

Next, a description will be given of the light beam that is used todetect the focal error signal FES in this embodiment. FIGS. 50A through50C are diagrams for explaining the shape of spots formed by the splitlight beams. FIG. 50A shows a plan view of the deflection surfaces 571,572 and 573 of the composite prism 507, together with light beams “a”and “b” when the two polarized light beams are transmitted through thedeflection surfaces 571,.572 and 573. FIGS. 50B and 50C respectivelyshow plan views of the photodetector part G of the photodetector unit509 on which the light beams “a” and “b” are projected, together withspots “aa” and “bb” of the light beams “a” and “b” irradiated on thephotodetector part G and the photodetector parts A and B via thedeflection surface 571. FIGS. 50B and 50C show the photodetector viewedfrom the back surface side of the photodetector unit 509.

The area of the light beam “b” transmitted through the deflectionsurface 571 is smaller than that of the light beam “a”, which means thatthe light beam “b” is deflected more. In this state, the length of thespot “bb” formed on the photodetector parts A and B along the major axisis longer than that of the spot “aa” formed on the photodetector part G.According to the Foucault technique, the longer the length of the spotalong the major axis, the easier: it is to perform the requiredadjustments of the arrangement of the optical elements and thephotodetectors, and the focal error signal FES is less affected bychanges with time, temperature changes and the like. Hence, in thisembodiment, the locations of the 2-part photodetector including thephotodetector parts A and B and the 2-part photodetector including thephotodetector parts C and D are determined so that out of the lightbeams transmitted through the deflection surfaces 571 and 573, the lightbeams which are deflected more are projected onto the 2-partphotodetector including the photodetector parts A and B and the 2-partphotodetector including the photodetector part C and D.

Next, a description will be given of the effects of the temperature onthis embodiment. When the temperature of the optical informationrecording/reproducing apparatus rises, a housing which fixes thephotodetector unit 509 undergoes a thermal expansion, thereby making thephotodetectors of the photodetector unit 509 farther away from thecomposite prism 507. For similar reasons, the objective lens 504 becomefarther away from the magneto-optic disk 505. As a result, the reflectedlight beam from the magneto-optic disk 505 is once converged andthereafter spreads before being projected onto the photodetector unit509. The spot formed on the 2-part photodetector including thephotodetector parts A and B and the spot formed on the 2-partphotodetector including the photodetector parts C and D are projected onthe corresponding division lines of the 2-part photodetectors when the2-part photodetectors are located at the focal positions of the lightbeams. However, if the 2-part photodetectors are located farther awayfrom the focal positions of the light beams, the spots of the lightbeams are formed on the outer sides of the corresponding division linesof the 2-part photodetectors as shown in FIG. 49, and in this case, thespots of the light beams are only formed on the photodetector parts Band D of the 2-part photodetectors.

When the temperature of the magneto-optic head rises, the separationangle of the light beams output from the Wollaston prism 506 alsochanges. FIG. 51 shows a plan view of the photodetector unit 509 at ahigh temperature,.viewed from the back surface side of the photodetectorunit 509. The Wollaston prism 506 is made of a birefringence materialsuch as crystal and lithium-niobate, and the separation angle of thelight beams changes depending on the temperature. For example, adescription will be given of the Wollaston prism 506 having a separationangle which becomes narrow at high temperatures. As shown in FIG. 51,the spots of the light beams formed on the 2-part photodetectors locatedon the right and left move towards the inner side of the photodetectorunit 509 due to the temperature characteristic of the Wollaston prism506. In other words, the separation angle of the light beams output fromthe Wollaston prism 506 becomes narrow due to the temperature rise, andthe spots of the light beams move toward the inner side of thephotodetector unit 509 to become projected on the corresponding divisionlines of the 2-part photodetectors respectively including thephotodetector parts A and B and the photodetector parts C and D.

In the case where the separation angle of the light beams output fromthe Wollaston prism 506 used becomes narrow at high temperatures and the2-part photodetectors located on the right and left of the photodetectorunit 509 are positioned farther away from the focal positions of thelight beams, the light beams used are such that the spots of the lightbeams are formed on the outer side of the corresponding division linesof the 2-part photodetectors. Hence, when the temperature rises and the2-part photodetectors become farther away from the Wollaston prism 506,the separation angle of the light beams output from the Wollaston prism506 becomes narrow such that the spots of the light beams are projectedon the corresponding division lines of the 2-part photodetectors of thephotodetector unit 509. As a result, it is possible to carry out astable automatic focusing operation based on an accurate focal errorsignal FES even when the temperature becomes high.

On the other hand, in the case where the separation angle of the lightbeams output from the Wollaston prism 506 used or, a Rochon prism (notshown) which is use in place of the Wollaston prism 506, becomes wide athigh temperatures and the 2-part photodetectors located on the right andleft of the photodetector unit 509 are positioned farther away from thefocal positions of the light beams, the light beams used are such thatthe spots of the light beams are formed on the inner side of thecorresponding division lines of the 2-part photodetectors. Hence, whenthe temperature rises and the 2-part photodetectors become farther awayfrom the Wollaston prism 506, the separation angle of the light beamsoutput from the Wollaston prism 506 becomes wide such that the spots ofthe light beams are projected on the corresponding division lines of the2-part photodetectors of the photodetector unit 509. As a result, it ispossible to carry out a stable automatic focusing operation based on anaccurate focal error signal FES even when the temperature becomes high.

Next, a description will be given of a first modification of thethirteenth embodiment of the optical information recording/reproducingapparatus according to the present invention, by referring to FIG. 52.In this first modification of the thirteenth embodiment, the basicconstruction of the optical information recording/reproducing apparatusis basically the same as that of the thirteenth embodiment shown inFIGS. 45 and 46, but a photodetector unit 591 is used in place of thephotodetector unit 509. FIG. 52 shows a plan view of the photodetectorunit 591 used in this first modification of the thirteenth embodiment.In FIG. 52, those parts which are the same as those corresponding partsin FIGS. 46 and 47 are designated by the same reference characters, anda description thereof will be omitted.

FIG. 52 shows the photodetector unit 591 in a state where the laser beamis in focus on the magneto-optic disk 505, viewed from the back surfaceside of the photodetector unit 591. The light beams reaching thephotodetector unit 591 via the Wollaston prism 506 and the compositeprism 507 is obtained via the same optical path as that of thethirteenth embodiment. Photodetectors of the photodetector unit 591 arearranged in three stages from the top to bottom in the verticaldirection. An upper stage portion of the photodetector unit 591 has aphotodetector including a photodetector part G for detecting amagneto-optic signal MO and an identification signal ID, and a 2-partphotodetector including photodetector parts AA and BB for detecting afocal error signal FES. A middle stage portion of the photodetector unit591 has a 2-part photodetector including photodetector parts E and F fordetecting a tracking error signal TES. Further, a lower stage portion ofthe photodetector unit 591 has a 2-part photodetector includingphotodetector parts C and D for detecting the focal error signal FES,and a photodetector including a photodetector part H for detecting themagneto-optic signal MO and the identification signal ID.

This first modification of the thirteenth embodiment, the light beamwhich forms a spot having an oval shape which is longer along the majoraxis and the light beam which forms a spot having an oval shape which isshorter along the major axis on the photodetector unit 591 are used toobtain the focal error signal FES. In other words, one of the two lightbeams obtained by the separation made at the Wollaston prism 506 orRochon prsim is used to detect the focal error signal FES. As a result,the obtained focal error signal FES is less affected by the temperaturecharacteristic of the Wollaston prism 506 or Rochon prism when comparedwith the thirteenth embodiment described above.

As described above, the separation angle of the two light beams outputfrom the Wollaston prism 506 or Rochon prsim changes with temperature.For example, the separation angle of the two light beams output from theWollaston prism 506 becomes narrow at high temperatures. Hence, if boththe two light beams separated and output from the Wollaston prism 506are used to detect the focal error signal FES, the focal error signalFES is greatly affected by the temperature. But if only one of the twolight beams separated and output from the Wollaston prism 506 is used todetect the focal error signal FES as in the case of the firstmodification of the thirteenth embodiment, it is possible to detect thefocal error signal FES without being affected by the temperaturecharacteristic of the Wollaston prism 506 or Rochon prism.

In this first modification of the thirteenth embodiment, if the outputsof the photodetector parts AA, BB and C through H of the photodetectorunit 591 are denoted by the same reference characters as these parts,the focal error signal FES according to the Foucault technique, thetracking error signal TES according to the push-pull technique, themagneto-optic signal MO and the identification signal ID can be obtainedbased on the following formulas (17) through (20a) or (20b) bycalculation, using known adders and subtracter.

FES=(AA+C)−(BB+D)   (17)

TES=E−F   (18)

MO=(AA+BB+C+D)−(G+H)   (19)

ID=(AA+BB+C+D)−(G+H)   (20a)

ID=AA+BB+C+D+E+F+G+H   (20b)

When the identification signal ID is reproduced based on the formula(20a), the number of adders provided in the LSI is relatively small,thereby making it possible to improve the S/N ratio of theidentification signal ID. On the other hand, the identification signalID may become distorted depending on the shape and depth of the pitsformed in the magneto-optic disk 505. Hence, when the identificationsignal ID is reproduced based on the formula (20b), it is possible topositively prevent the identification signal ID from becoming distortedbecause all of the diffracted light beams from the pits of themagneto-optic disk 505 are used to detect the identification signal ID.

Next, a description will be given of a second modification of thethirteenth embodiment of the optical information recording/reproducingapparatus according to the present invention, by referring to FIG. 53.In this second modification of the thirteenth embodiment, the basicconstruction of the optical information recording/reproducing apparatusis basically the same as that of the thirteenth embodiment shown inFIGS. 45 and 46, but a photodetector unit 592 is used in place of thephotodetector unit 509. FIG. 53 shows a plan view of the photodetectorunit 592 used in this second modification of the thirteenth embodiment.In FIG. 53, those parts which are the same as those corresponding partsin FIGS. 46 and 47 are designated by the same reference characters, anda description thereof will be omitted.

FIG. 53 shows the photodetector unit 592 in a state where the laser beamis in focus on the magneto-optic disk 505, viewed from the back surfaceside of the photodetector unit 592. The light beams reaching thephotodetector unit 592 via the Wollaston prism 506 and the compositeprism 507 is obtained via the same optical path as that of thethirteenth embodiment. Photodetectors of the photodetector unit 592 arearranged in three stages from the top to bottom in the verticaldirection. An upper stage portion of the photodetector unit 592 has aphotodetector including a photodetector part G for detecting amagneto-optic signal MO and an identification signal ID, and a 2-partphotodetector including photodetector parts A and B for detecting afocal error signal FES. A middle stage portion of the photodetector unit592 has a 2-part photodetector including photodetector parts El and F1and a 2-part photodetector including photodetector parts E2 and F2 fordetecting a tracking error signal TES. Further, a lower stage portion ofthe photodetector unit 592 has a 2-part photodetector includingphotodetector parts C and D for detecting the focal error signal FES,and a photodetector including a photodetector part H for detecting themagneto-optic signal MO and the identification signal ID.

The 2-part photodetectors respectively including the photodetector partsE1 and F1 and the photodetector parts E2 and F2 receive the light beamsoutput from the deflection surface 572 of the composite prism 507. Ifthe outputs of the photodetector parts A through D, E1, E2, F1, F2, Gand H of the photodetector unit 592 are denoted by the same referencecharacters as these parts, the focal error signal FES according to theFoucault technique, the tracking error signal TES according to thepush-pull technique, the magneto-optic signal MO and the identificationsignal ID can be obtained based on the following formulas (21a) through(24a) or (24a′) by calculation using known adders and subtracter.

FES=(A+C)−(B+D)   (21a)

TES=(E 1+E 2)−(F 1+F 2)   (22a)

MO=(E 1+F 1+G)−(E 2+F 2+H)   (23a)

ID=(E 1+F 1+G)+(E 2+F 2+H)   (24a)

ID=A+B+C+D+E 1+E 2+F 1+F 2+G+H   (24a′)

When the identification signal ID is reproduced based on the formula(24a), the number of adders provided in the LSI is relatively small,thereby making it possible to improve the S/N ratio of theidentification signal ID. On the other hand, the identification signalID may become distorted depending on the shape and depth of the pitsformed in the magneto-optic disk 505. Hence, when the identificationsignal ID is reproduced based on the formula (24a′), it is possible topositively prevent the identification signal ID from becoming distortedbecause all of the diffracted light beams from the pits of themagneto-optic disk 505 are used to detect the identification signal ID.

According to this second modification of the thirteenth embodiment, themagneto-optic signal MO and the identification signal ID are reproducedusing the light beams which are used to detect the tracking error signalTES.

Alternatively, it is possible to reproduce the magneto-optic signal MOand the identification signal ID using the light beams which are used todetect both the tracking error signal TES and the focal error signalFES. In this case, the focal error signal FES according to the Foucaulttechnique, the tracking error signal TES according to the push-pulltechnique, the magneto-optic signal MO and the identification signal IDcan be obtained based on the following formulas (21b) through (24b) bycalculation using known adders and subtracter.

FES=(A+C)−(B+D)   (21b)

TES=(E+E 2)−(F 1+F 2)   (22b)

MO=(A+B+E 2+F 2+H)−(C+D+E 1+F 1+G)   (23b)

ID=(A+B+E 2+F 2+H)+(C+D+E 1+F 1+G)   (24b)

When the identification signal ID is reproduced based on the formula(24b), the number of adders provided in the LSI is relatively small,thereby making it possible to improve the S/N ratio of theidentification signal ID.

Next, a description will be given of a third modification of thethirteenth embodiment of the optical information recording/reproducingapparatus according to the present invention, by referring to FIGS. 54through 56. In FIG. 54, those parts which are the same as thosecorresponding parts in FIG. 46 and 53 are designated by the samereference numerals, and a description thereof will be omitted.

In this third modification of the thirteenth embodiment, the analyzer506 is used together with a composite prism 547 shown in FIG. 55 and thephotodetector unit 592 shown in FIG. 54.

The reflected light beam from the magneto-optic disk 505 is split intotwo light beams by the analyzer 506, and each of the two light beams arefurther split into five light beams by the composite prism 547, therebyresulting in ten (2×5=10) light beams being output from the compositeprism 547. The ten light beams from the composite prism 547 areirradiated on corresponding ones of six photodetectors which form thephotodetector unit 592.

FIG. 55 shows a perspective view of the composite prism 547. As shown inFIG. 55, the composite prism 547 includes tapered first and second parts547-1 and 547-2, a central third part 547-3 which has a convex surfacewith a slight curvature, and peripheral fourth and fifth parts 547-4 and547-5 which have convex surfaces with a slight curvature matching thatof the third part 547-3. In other words, the third, fourth and fifthparts 547-3, 547-4 and 547-5 are all parts of a single convex surfacehaving a slight curvature. The first and second parts 547-1 and 547-2function similarly to the first and second parts 571 and 573 of thecomposite prism 507.

FIG. 56 shows a plan view of the photodetector unit 592. As shown inFIG. 56, the photodetector unit 592 includes the photodetectorsrespectively including the photodetector parts G and H, and the 2-partphotodetectors respectively including the photodetector parts A and B, Cand D, E1 and F2, and E2 and F2.

The two light beams output from the third part 547-3 of the compositeprism 547 are respectively irradiated on the 2-part photodetectorincluding the photodetector parts E1 and F1 and the 2-part photodetectorincluding the photodetector parts E2 and F2. The two light beams outputfrom the fourth part 547-4 of the composite prism 547 are respectivelyirradiated on the 2-part photodetector including the photodetector partsE1 and F1 and the 2-part photodetector including the photodetector partsE2 and. F2. Further, the two light beams output from the fifth part547-5 of the composite prism 547 are respectively irradiated on the2-part photodetector including the photodetector parts E1 and F1 and the2-part photodetector including the photodetector parts E2 and F2. On theother hand, the two light beams output from the first part 547-1 of thecomposite prism 547 are respectively irradiated on the photodetectorincluding the photodetector part G and the 2-part photodetectorincluding the photodetector parts A and B. The two light beams outputfrom the second part 547-2 of the composite prism 547 are respectivelyirradiated on the 2-part photodetector including the photodetectors Cand D and the photodetector including the photodetector part H.

The image formation points of the four light beams output from the firstand second parts 547-1 and 547-2 match the positions of thecorresponding photodetectors in the upper and lower stages of thephotodetector unit 592. On the other hand, the image formation points ofthe two light beams output from each of the third, fourth and fifthparts 547-3, 547-4 and 547-5 are deviated from the positions of thecorresponding 2-part photodetectors in the middle stage of thephotodetector unit 592.

If the outputs of the photodetector parts A through D, E1, E2, F1, F2, Gand H of the photodetector unit 592 are denoted by the same referencecharacters as these parts, the focal error signal FES using the Foucaulttechnique, the tracking error signal TES using the push-pull technique,the magneto-optic signal MO, and the identification signal ID can beobtained by calculations based on the formulas (25) through (28a) or(28b) using known adders and subtracter.

FES=(A+C)−(B+D)   (25)

TES=(E 1+E 2)−(F 1+F 2)   (26)

MO=(E 1+F 1)−(E 2+F 2)   (27)

ID=(E 1+F 1)+(E 2+F 2)   (28a)

ID=A+B+C+D+E 1+E 2+F 1+F 2+G+H   (28b)

When the identification signal ID is reproduced based on the formula(28a), the number of adders provided in the LSI is relatively small,thereby making it possible to improve the S/N ratio of theidentification signal ID. On the other hand, the identification signalID may become distorted depending on the shape and depth of the pitsformed in the magneto-optic disk 505. Hence, when the identificationsignal ID is reproduced based on the formula ( 28 b), it is possible topositively prevent the identification signal ID from becoming distortedbecause all of the diffracted light beams from the pits of themagneto-optic disk 505 are used to detect the identification signal ID.

According to the magneto-optic signal MO obtained by the formula (27),it is possible to obtain a relatively high resolution. The reason forthis further improved resolution of the magneto-optic signal MO usingthe composite prism 547 having the shape shown in FIG. 55 may beunderstood from the teachings of the Proceedings of Magneto-OpticalRecording International Symposium '96, J. Magn. Soc. Jpn., Vol.20,Supplement No.S1 (1996), pp.233-238.

Next, a description will be given of a fourteenth embodiment of theoptical information recording/reproducing apparatus according to thepresent invention, by referring to FIG. 57. FIG. 57 shows a perspectiveview of an important part of the fourteenth embodiment of the opticalinformation recording/reproducing apparatus, that is, a signal detectionsystem, on an enlarged scale. In FIG. 57, those parts which are the sameas those corresponding parts in FIG. 46 are designated by the samereference numerals, and a description thereof will be omitted.

FIG. 57 shows a state where the light beam output from the Wollastonprism 506 is projected on the photodetector unit 509. The reflectedlight beam input to the Wollaston prism 506 is separated into twopolarized light beams and input to a composite prism 517. As shown inFIG. 57, the composite prism 517 is an optical element having threedeflection surfaces 571, 572 a and 573 arranged side by side from theleft to right. The deflection surfaces 571 and 573 on both sides of thecomposite prism 517 have tapered surfaces for deflecting the light beamin upward and downward directions by mutually different angles. On theother hand, the deflection surface 572 a at the central part of thecomposite prism 517 has a concave shape having a curvature with respectto the optical axis, and directs the two split light beams towards thephotodetector parts E and F of the 2-part photodetector forming thephotodetector unit 509. Each of the two polarized light beams input tothe composite prism 517 having the structure described above isspatially split into three by the three deflection surfaces 571, 572 aand 573, thereby outputting a total of six light beams from thecomposite prism 517. The six light beams output from the composite prism517 are projected on the photodetector unit 509 via a condenser lens508. For the sake of convenience, the illustration of the condenser lens508 shown in FIG. 45 is omitted in FIG. 57.

Otherwise, the functions and effects of this embodiment are similar tothose of the thirteenth embodiment described above, and the focal errorsignal FES, the tracking error signal TES, the magneto-optic signal MOand the identification signal ID can be obtained based on the formulasdescribed with reference to the thirteenth embodiment. Furthermore, thisembodiment may employ the photodetector unit 591 or 592 of the first orsecond modification of the thirteenth embodiment described above, so asto obtain similar functions and effects as those of the first or secondmodification of the thirteenth embodiment.

Next, a description will be given of a fifteenth embodiment of theoptical information recording/reproducing apparatus according to thepresent invention, by referring to FIG. 58. FIG. 58 shows a perspectiveview of an important part of the fifteenth embodiment of the opticalinformation recording/reproducing apparatus, that is, a signal detectionsystem, on an enlarged scale. In FIG. 58, those parts which are the sameas those corresponding parts in FIG. 46 are designated by the samereference numerals, and a description thereof will be omitted.

FIG. 58 shows a state where the light beam output from the Wollastonprism 506 is projected on a photodetector unit 593. The reflected lightbeam input to the Wollaston prism 506 is separated into two polarizedlight beams and input to a composite prism 527. As shown in FIG. 58, thecomposite prism 527 is an optical element having three deflectionsurfaces 571 b, 572 b and 573 b arranged side by side from the left toright. The deflection surfaces 571 b and 573 b on both sides of thecomposite prism 527 have tapered surfaces for deflecting the light beamin upward and downward directions by mutually different angles. On theother hand, the deflection surface 572 b at the central part of thecomposite prism 527 has a flat surface which is perpendicular to theoptical axis, and directs the two split light beams towards thephotodetector parts E and F of a 2-part photodetector forming thephotodetector unit 593. Each of the two polarized light beams input tothe composite prism 527 having the structure described above isspatially split into three by the three deflection surfaces 571 b, 572 band 573 b, thereby outputting a total of six light beams from thecomposite prism 527. The six light beams output from the composite prism527 are projected on the photodetector unit 593 via a condenser lens508. For the sake of convenience, the illustration of the condenser lens508 shown in FIG. 45 is omitted in FIG. 58.

The photodetector unit 593 has a stepped shape such that the 2-partphotodetector in the middle stage and including the photodetector partsE and F is arranged closer to the composite prism 527 than thephotodetectors in the upper and lower stages of the photodetector unit593.

Otherwise, the functions and effects of this embodiment are similar tothose of the thirteenth embodiment described above, and the focal errorsignal FES, the tracking error signal TES, the magneto-optic signal MOand the identification signal ID can be obtained based on the formulasdescribed with reference to the thirteenth embodiment. Furthermore, thisembodiment may employ the photodetector unit 591 or 592 of the first orsecond modification of the thirteenth embodiment described above bymodifying the photodetector 591 or 592 to have stepped shape similar tothat of the photodetector 593, so as to obtain similar functions andeffects as those of the first or second modification of the thirteenthembodiment.

Of course, the photodetector unit 593 may have a stepped shape such thatthe 2-part photodetector in the middle stage and including thephotodetector parts E and F is arranged farther away from the compositeprism 527 than the photodetectors in the upper and lower stages of thephotodetector unit 593.

Next, a description will be given of a sixteenth embodiment of theoptical information recording/reproducing apparatus according to thepresent invention, by referring to FIG. 59. FIG. 59 shows a perspectiveview of an important part of the sixteenth embodiment of the opticalinformation recording/reproducing apparatus, that is, a signal detectionsystem, on an enlarged scale. In FIG. 59, those parts which are the sameas those corresponding parts in FIG. 46 are designated by the samereference numerals, and a description thereof will be omitted.

FIG. 59 shows a state where the light beam output from the Wollastonprism 506 is projected on a photodetector unit 593. The reflected lightbeam input to the Wollaston prism 506 is separated into two polarizedlight beams and input to a holographic optical element 537. As shown inFIG. 59, the holographic optical element 537 is an optical elementhaving two diffraction surfaces 537 a and 537 b arranged side by sidefrom to each other. The diffraction surfaces 537 a and 537 b of theholographic optical element 537 have gratings with sawtooth-shaped crosssections which are provided symmetrically about a center point of theholographic optical element 537.

The two polarized light beams input to the holographic optical element537 are respectively separated mainly into ±1st and 0th order diffractedlight beams by the diffraction surfaces 527 a and 537 b, and a total ofsix light beams are projected onto the corresponding photodetectors ofthe photodetector unit 509 via a condenser lens 508. Actually, highorder diffracted light beams of ±2nd order or greater are generated fromthe holographic optical element 537, but the light quantity of the highorder diffracted light beams is small and negligible. For the sake ofconvenience, the illustration of the condenser lens 508 shown in FIG. 45is omitted in FIG. 59.

Otherwise, the functions and effects of this embodiment are similar tothose of the thirteenth embodiment described above, and the focal errorsignal FES, the tracking error signal TES, the magneto-optic signal MOand the identification signal ID can be obtained based on the formulasdescribed with reference to the thirteenth embodiment. Furthermore, thisembodiment may employ the photodetector unit 591 or 592 of the first orsecond modification of the thirteenth embodiment described above, so asto obtain similar functions and effects as those of the first or secondmodification of the thirteenth embodiment.

In the thirteenth through sixteenth embodiments and the modificationsthereof, it is of course possible to arrange the composite prism 507,547, 517 or 527 or the holographic optical element 537, the condenserlens 508, and the analyzer 506 in an arbitrary order. In other words,the analyzer and the composite prism or holographic optical element maybe arranged in an arbitrary order in the optical path for directing thereflected light beam from the optical recording medium to thephotodetector unit.

FIGS. 60A through 60D respectively are circuit diagrams showing circuitsfor obtaining the focal error signal FES, the tracking error signal TES,the magneto-optic signal MO and the identification signal ID of thefirst modification of the eighth embodiment based on the formulas (7)through (10) described above.

FIG. 60A shows a circuit including adders 801 and 802 and a subtracter803 which are connected as shown to generate the focal error signal FESbased on the outputs of the photodetector parts A through D.

FIG. 60B shows a circuit including a subtracter 804 to generate thetracking error signal TES based on the outputs of the photodetectorparts E and F.

FIG. 60C shows a circuit including a subtracter 805 to reproduce themagneto-optic signal MO based on the outputs of the photodetector partsG and G.

FIG. 60D shows a circuit including an adder 806 to reproduce theidentification signal ID based on the outputs of the photodetector partsG and H.

The circuits for obtaining the focal error signal FES, the trackingerror signal TES, the magneto-optic signal MO and the identificationsignal ID based on other formulas described above can similarly beconstructed using known adders and subtracters, and the illustration ofsuch circuits will be omitted in this application since the connectionsof the adders and subtracters are evident from the formulas.

FIG. 61 is a perspective view showing a composite prism which may beused in place of the composite prisms 35B and 547 shown in FIGS. 30 and55.

As shown in FIG. 61, a composite prism 135B includes tapered first andsecond parts 135B-1 and 135B-2, a central third part 135B-3 which has aconcave surface with a slight curvature, and peripheral fourth and fifthparts 135B-4 and 135B-5 which have concave surfaces with a slightcurvature matching that of the third part 135B-3. In other words, thethird, fourth and fifth parts 135B-3, 135B-4 and 135B-5 are all parts ofa single concave surface having a slight curvature. The first and secondparts 135B-1 and 135B-2 function similarly to the first and second parts35B-1 and 35B-2 of the composite prism 35B shown in FIG. 30. Thiscomposite prism 135B may be used in the optical system shown in FIGS. 29and 54, for example, and substantially the same effects are obtainableas in the embodiments shown in FIGS. 29 and 54.

Further, the present invention is not limited to these embodiments, butvarious variations and modifications may be made without departing fromthe scope of the present invention.

What is claimed is:
 1. An optical information recording/reproducingapparatus which records information on and/or reproduces informationfrom an optical recording medium based on a reflected light beam fromthe optical recording medium, said optical informationrecording/reproducing apparatus comprising: a light splitting parthaving a plurality of light splitting stages which split the reflectedlight beam from the optical recording medium into a plurality of lightbeams; and a photodetector unit including a plurality of photodetectorswhich receive the plurality of light beams from said light splittingpart, said photodetector unit including a first plurality ofphotodetectors which receive light beams used to detect a focal error,at least one photodetector which receives a light beam used to detect atracking error, and a second plurality of photodetectors which receivelight beams used to detect optical information recorded on the opticalrecording medium, wherein said at least one photodetector which receivesa light beam used to detect the tracking error is not included in saidfirst plurality of photodetectors, said light splitting part includingan analyzer which splits an incoming light beam into polarized lightbeams and a composite prism which spatially splits an incoming lightbeam into a plurality of light beams, said analyzer and said compositeprism being arranged in an arbitrary order between the optical recordingmedium and said photodetector unit.
 2. The optical informationrecording/reproducing apparatus as claimed in claim 1, wherein saidlight splitting part includes an analyzer which splits an incoming lightbeam into polarized light beams and a composite prism which spatiallysplits an incoming light beam into a plurality of light beams, saidanalyzer and said composite prism being arranged in an arbitrary orderbetween the optical recording medium and said photodetector unit.
 3. Theoptical information recording/reproducing apparatus as claimed in claim1, wherein said photodetector unit includes a plurality ofphotodetectors which detect positional information, said positionalinformation being prerecorded on the optical recording medium andindicating a position on the optical recording medium.
 4. The opticalinformation recording/reproducing apparatus as claimed in claim 3,wherein the plurality of photodetectors which detect the positionalinformation are the same as the second plurality of photodetectors whichdetect the optical information.
 5. The optical informationrecording/reproducing apparatus as claimed in claim 1, wherein thesecond plurality of photodetectors which receive the light beams used todetect the optical information only receive selected ones of theplurality of light beams output from said light splitting part.
 6. Anoptical information recording/-reproducing apparatus which recordsinformation on and/or reproduces information from an optical recordingmedium based on a reflected light beam from the optical recordingmedium, said optical information recording/reproducing apparatuscomprising: a light splitting part having a plurality of light splittingstages which split the reflected light beam from the optical recordingmedium into a plurality of light beams; and a photodetector unitincluding a plurality of photodetectors which receive the plurality oflight beams from said light splitting part, said photodetector unitincluding a first plurality of photodetectors which receive light beamsused to detect a focal error, at least one photodetector which receivesa light beam used to detect a tracking error, and a second plurality ofphotodetectors which receive light beams used to detect opticalinformation recorded on the optical recording medium, wherein said lightsplitting part splits the reflected light beam from the opticalrecording medium into nine light beams, and said photodetector unitincludes five photodetectors which receive the nine light beams.
 7. Anoptical information recording/reproducing apparatus which recordsinformation on and/or reproduces information from an optical recordingmedium based on a reflected light beam from the optical recordingmedium, comprising: a light splitting part having a plurality of lightsplitting stages which split the reflected light beam from the opticalrecording medium into a plurality of light beams; and a photodetectorunit including n photodetectors which receive m light beams output froma final stage of said light splitting part, where m and n are positiveintegers satisfying m≧n, said n photodetectors of said photodetectorunit including a first plurality of photodetectors which are arranged toreceive light beams used to detect a focal error, at least onephotodetector which is arranged to receive a light beam used to detect atracking error, and a second plurality of photodetectors which arearranged to receive light beams used to detect optical informationrecorded on the optical recording medium, wherein said at least onephotodetector which receives the light beam used to detect the trackingerror is not included in said first plurality of photodetectors, saidlight splitting part including an analyzer which splits an incominglight beam into polarized light beams and a composite prism whichspatially splits an incoming light beam into a plurality of light beams,said analyzer and said composite prism being arranged in an arbitraryorder between the optical recording medium and said photodetector unit.8. An optical information recording/reproducing apparatus which recordsinformation on and/or reproduces information from an optical recordingmedium based on a reflected light beam from the optical recordingmedium, comprising: a light splitting part having a plurality of lightsplitting stages which split the reflected light beam from the opticalrecording medium into a plurality of light beams; and a photodetectorunit including n photodetectors which receive m light beams output froma last stage of said light splitting part, where m and n are positiveintegers satisfying m>n, said n photodetectors of said photodetectorunit including a plurality of photodetectors which are arranged toreceive light beams used to detect a focal error, at least onephotodetector which is arranged to receive a light beam used to detect atracking error, and a plurality of photodetectors which are arranged toreceive in common light beams used to detect optical informationrecorded on the optical recording medium, said integers m and n beingselected to satisfy one of the conditions m=9 and n=5, and m=15 and n=5.9. The optical information recording/reproducing apparatus as claimed inclaim 8, wherein: the photodetectors which are arranged to receive thelight beams used to detect the focal error include two two-partphotodetectors respectively having photodetector parts A and B andphotodetector parts C and D; and which further comprises: means forproducing a focal error signal FES based on outputs of the two-partphotodetectors by a formula: FES=(A+C)−(B+D), where the outputs of thephotodetector parts are designated by the same designations as thephotodetector parts.
 10. The optical information recording/reproducingapparatus as claimed in claim 8, wherein: the photodetector which isarranged to receive the light beams used to detect the focal errorincludes a two-part photodetectors having photodetector parts E and F;and which further comprises: means for producing a tracking error signalTES based on outputs of the 2-part photodetector by a formula: TES=E−F,where the outputs of the photodetector parts are designated by the samedesignations as the photodetector parts.
 11. The optical informationrecording/reproducing apparatus as claimed in claim 8, wherein: thephotodetectors which are arranged to receive the light beams used todetect the optical information include two photodetector parts G and H;and which further comprises: means for producing an optical signal MObased on outputs of the photodetectors by a formula: MO=G−H, where theoutputs of the photodetector parts are designated by the samedesignations as the photodetector parts.
 12. The optical informationrecording/reproducing apparatus as claimed in claim 8, wherein: thephotodetectors which are arranged to receive the light beams used todetect the optical information include two photodetector parts G and H;and which further comprises: means for producing an identificationsignal ID based on outputs of the photodetectors by a formula: ID=G+H,where the outputs of the photodetector parts are designated by the samedesignations as the photodetector parts.
 13. The optical informationrecording/reproducing apparatus as claimed in claim 8, wherein: thephotodetectors which are arranged to receive the light beams used todetect the focal error include two two-part photodetectors respectivelyhaving photodetector parts A and B and photodetector parts C and D; thephotodetector which is arranged to receive the light beams used todetect the tracking error signal includes one two-part photodetectorhaving photodetector parts E and F; the photodetectors which arearranged to receive the light beam used to detect optical informationinclude two photodetectors G and H; and which further comprises: meansfor producing a focal error signal, a tracking error signal, an opticalsignal and an identification signal based on outputs of thephotodetector parts.
 14. An optical information recording/reproducingapparatus which records information on and/or reproduces informationfrom an optical recording medium based on a reflected light beam fromthe optical recording medium, said optical informationrecording/reproducing apparatus comprising: an analyzer; a compositeprism; and a photodetector unit, all of which are arranged approximatelylinearly along a common optical axis of the reflected light beam, saidphotodetector unit including a plurality of photodetectors, wherein saidanalyzer has a function of splitting the reflected light beam into aplurality of light beams having different polarization directions, saidcomposite prism has a plurality of deflection parts for spatiallysplitting each of the plurality of light beams received from saidanalyzer into three light beams, said photodetector unit receives thelight beams deflected by said composite prism and produces a focal errorsignal, a tracking error signal and an optical signal based on the lightbeams received by the plurality of photodetectors, and the photodetectorused for receiving the tracking error signal is not used for producingthe focal error signal or the optical signal.